The Chronobiology, Etiology and Pathophysiology of Obesity

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International Journal of Obesity (2010) 34, 1667–1683 & 2010 Macmillan Publishers Limited All rights reserved 0307-0565/10 www.nature.com/ijo REVIEW The chronobiology, etiology and pathophysiology of obesity

M Garaulet1, JM Ordova´s2 and JA Madrid1

1Faculty of Biology, Department of Physiology, Campus of Espinardo, University of Murcia, Murcia, Spain and 2Jean Mayer US Department of Agriculture Human Nutrition Research Center on Aging, Tufts University School of Medicine, Boston, MA, USA

The effect of CD on human health is an emerging issue. Many records link CD with diseases such as cancer, cardiovascular, cognitive impairment and obesity, all of them conducive to premature aging. The amount of sleep has declined by 1.5 h over the past century, accompanied by an important increase in obesity. Shift work, sleep deprivation and exposure to bright light at night increase the prevalence of adiposity. Animal models have shown that mice with Clock gene disruption are prone to developing obesity and MetS. This review summarizes the latest developments with regard to chronobiology and obesity, considering (1) how molecular clocks coordinate metabolism and the specific role of the adipocyte; (2) CD and its causes and pathological consequences; (3) the epidemiological evidence of obesity as a chronobiological illness; and (4) theories of circadian disruption and obesity. Energy intake and expenditure, relevance of sleep, fat intake from a circadian perspective and psychological and genetic aspects of obesity are examined. Finally, ideas about the use of chronobiology in the treatment of obesity are discussed. Such knowledge has the potential to become a valuable tool in the understanding of the relationship between the chronobiology, etiology and pathophysiology of obesity. International Journal of Obesity (2010) 34, 1667–1683; doi:10.1038/ijo.2010.118; published online 22 June 2010

Keywords: metabolic syndrome; chronobiology; circadian; clock genes

Introduction mortality associated with obesity continue to increase. Endogenous (that is, genetic) and exogenous factors Correct timing of the endogenous circadian clock enables (that is, diet and physical activity) have an important role organisms to predict and anticipate daily environmental in the assessment and management of obesity.2 Recently, changes, and temporally adjust behavioral and physiological studies have suggested that disruption of the circadian functions accordingly. In humans, current societal habits, system (chronodisruption (CD)) may lead to obesity.3 CD including high snacking frequency, a reduction in the time can also be produced by alteration of the core machinery of spent sleeping and increased exposure to bright light during the molecular circadian clock. Brain- and muscle ANRT-like the night, act on the brain to induce a loss of ‘feeling’ for protein-1 (BMAL1), Period Circadian Protein-2 (PER2) and internal and external rhythms. Consequently, the environ- Circadian Locomotor Output Cycles Kaput (CLOCK), among ment sensed by the brain has become metabolically flattened others clock proteins, have a specific role in the organism’s and arrhythmic. Among the well-known consequences of physiology in addition to their role in the circadian a disrupted circadian function are altered metabolism and molecular clock. Animal models have shown that mice with even life span, which may be all adversely affected when the Clock gene disruption are prone to developing obesity and circadian time-keeping system is altered.1 a phenotype resembling metabolic syndrome (MetS).4 Obesity has emerged as one of the most serious public Complications of obesity include cardiovascular disease, health concerns in the 21st century and the morbidity and hypertension, dyslipidemia, endothelial dysfunction, type-2 diabetes mellitus and impaired glucose tolerance, among others. Interestingly, chronobiology is implicated in most of Correspondence: Dr M Garaulet, Faculty of Biology, Department of these alterations.5 Circadian control of the cardiovascular Physiology, University of Murcia, Faculty of Biology, Campus of Espinardo, function is firmly established. Furthemore, it is known that Murcia 30100, Spain. many hormones involved in metabolism, such as insulin, E-mail: [email protected] Received 30 September 2009; revised 16 April 2010; accepted 18 April 2010; glucagon, growth hormone and cortisol, show circadian published online 22 June 2010 oscillation with different daily patterns. Current studies are Clock genes and obesity M Garaulet et al 1668 illustrating the particular role of different clock gene variants However, under natural environmental conditions, the and their predicted haplotypes in obesity. This review will SCN is reset every day by the periodical light/dark signal summarize recent findings concerning the relationship through a non-visual pathway consisting of the melanopsin between the chronobiology, etiology and pathophysiology ganglionar cells and the retinohypothalamic tract. Although of obesity. the photic input is the main SCN entraining signal, other periodical cues, such as feeding time, social contacts and regular physical activity, can also entrain the mammalian circadian system.6,7 How molecular clocks coordinate metabolism When food availability is limited to short time periods (restricted feeding) many circadian rhythms can phase- The mammalian circadian time system is composed of a shift to adapt the organism’s physiology to the time of network of hierarchically organized structures responsible scheduled food (food-anticipatory activity, FAA). However, for the generation of circadian rhythms and their synchro- under unrestricted access to food, when light becomes the nization with the environment. This circadian system dominant synchronizing signal, these circadian rhythms can be conceptualized as having three sets of components: return to their previous phase.8 Evidence from SCN lessoned the inputs, the 24-h oscillators and the outputs (Figure 1). It animals, which are able to maintain circadian rhythmicity is essential to note that the most important circadian entrained to restricted feeding, suggests that in addition to oscillator is a central pacemaker, located in the suprachia- the light-entrainable pacemaker (SCN) another circadian smatic nucleus (SCN) of the hypothalamus. oscillator may supersedes the SCN.8 This putative clock, Because the endogenous SCN oscillation period is called the feeding-entrainable oscillator, could be located not exactly 24 h, circadian rhythms free-run with a period in the dorsomedial nucleus of the hypothalamus (DMH), slightly different from 24 h when the subject is maintained as only insertion of functional Bmal1, an essential clock under artificially constant environmental conditions. component, in the DMH neurons restores the ability of

INPUTS CENTRAL PACEMAKERS OUTPUTS Pineal

Sleep/wake TGFα, EGF Melatonin Prokineticin 2 SCN Light/dark Glutamate Nocturnin Locomotor PACAP Melatonin activity

Social Endocrine Contacts rhythms Glucocorticoids Sympathetic Parasympathetic Body Physical temperature exercise

Cardiovascular rhythms Food

Feeding time PERIPHERAL OSCILLATORS

Figure 1 A general overview of the functional organization of the circadian system in mammals. Inputs: Environmental periodical cues can reset the phase of the central pacemaker so that the period and phase of circadian rhythms became coincident with the timing of the external cues. Central pacemakers: The SCNis considered the major pacemaker of the circadian system, driving circadian rhythmicity in other brain areas and peripheral tissues by sending them neural and humoral signals. Peripheral oscillators: Most peripheral tissues and organs contain circadian oscillators. Usually they are under the control of the SCN; however, under some circumstances (that is, restricted feeding, jet-lag and shift work.), they can desynchronize from the SCN. Outputs: Central pacemakers and peripheral oscillators are responsible for the daily rhythmicity observed in most physiological and behavioral functions. Some of these over-rhythms (physical exercise, core temperature, sleep–wake cycle and feeding time), in turn, provide a feedback, which can modify the function of SCN and peripheral oscillators. SCN, suprachiasmatic nucleus.

International Journal of Obesity Clock genes and obesity M Garaulet et al 1669 Bmal1-defective animals to entrain to food but not light. and negative transcriptional/translational feedback loops. Similarly, insertion of Bmal1 in the SCN was able to restore the Although the molecular clock model is continuously evol- light but not food-entrained rhythms.9 However, the persis- ving, the current view suggests that the core of the molecular tence of FAA in DMH lesioned rats9 suggests that feeding- clock is composed of two basic helix–loop–helix PER-ARNT- entrainable oscillator could be dependent on a network of SIM (Period circadian protein–Aryl hydrocarbon Receptor oscillators, rather than be located in one place. In intact Nuclear Translocator protein–SIngle-Minded protein) tran- animals, different circadian oscillators located in the digestive scription factors, CLOCK (circadian locomotor output cycles tract, liver or cerebral areas other than the SCN might kaput) and BMAL1 (brain- and muscle- ANRT-like protein), contribute to entraining the circadian rhythms of feeding which act as positive elements in the feed back loop times, raising strong expectations that one or more structures (Figure 2). Recently, it has been proposed that the CLOCK that underlie FAA.10 Recently, a new discovery has increased homolog neuronal PAS domain (name derived from PER- the present controversy.11 Mutant mice lacking known ARNT-SIM) protein-2 (NPAS2) could functionally compen- circadian clock function in all tissues show normal FAA both sate for the lack of CLOCK. The heterodimer CLOCK–BMAL1 in a light–dark cycle and in constant darkness, regardless of binds to E-boxes in the promoter of some target genes, whether the mutation disables the positive or negative limb of driving the transcription of six repressor encoding genes: the clock feedback mechanism. FAA seems to be independent three period genes (Per1, Per2 and Per3), two cryptochrome of the known circadian clock. The authors concluded that (Cry1 and Cry2) and the transcription factor reverse erythro- either FAA is not the output of an oscillator or that it is the blastosis virus-a (Rev-Erba) gene, and one promoting gene, output of a circadian oscillator different from known circadian Ror-a (retinoic acid receptor-related orphan receptor-a) clocks. New research is needed to confirm these results and to (Figure 2). PER and cryptochrome circadian protein (CRY) show the existence of a real feeding-entrainable oscillator. dimerize and are then translocated into the nucleus. The In addition to the SCN, other brain areas have been transcription of the repressor-encoding genes is stopped proposed as self-sustained circadian oscillators and challenge when PER–CRY dimers counteract the positive effect of the traditional uniclock model. Among these areas, the CLOCK–BMAL1 and thereby inhibit their own transcription. retina and the olfactory bulb are master oscillators, which are In addition, the Bmal1 rhythm is regulated by rhythmical capable of self-sustained circadian output in isolation.12 changes in RORE occupancy by REV-ERBa and RORa A crucial step toward a better understanding of circadian (Figure 2). Other nuclear receptors and coactivators, such time system physiology was the discovery that circadian as peroxisome proliferator-activated receptors (PPARs) clocks are present in most peripheral tissues13 (Figure 1). Self- and PPARg-coactivator-1a (PGCa), have been proposed as sustained oscillations can be observed in some organs and modulators of BMAL1 and CLOCK (Figure 2). tissues such as the heart, lung, liver, intestine and adrenal It has been shown that PPARa can modify BMAL1 and adipose tissue (AT). These peripheral oscillators must and CLOCK expression and activity by its union to PPAR- receive periodical inputs from the SCN to prevent the response elements located in the Bmal1 promoter.14–17 spontaneous dampening of their rhythmical activity with Similarly, PPARa induces the expression of rev-ERa by its time. However, they are also sensitive to their own union to PPAR-response elements in its promoter.18,19 synchronizers such as feeding time, local temperature, Conversely, PPARg-coactivator-1a (PGC-1a) induces the glucocorticoids, retinoic acid, prostaglandins, adrenaline, transcription of Bmal1 and Rev-Erba through coactivation of noradrenaline, glucose and angiotensin-II. de RORa.20 PGC-1a is the key factor in the homeostasis of In contrast to the relatively well-known structure and func- glucose, lipids, energy and it is also modulated by stress and tion of photic synchronizing inputs and the master circadian nutrients.21,22 Indeed, PGC-1a-knockout mice show meta- pacemaker, the pathways acting downstream from the SCN bolic alterations (disrupted weight control, muscle dysfunc- remain poorly understood. These pathways are used by the SCN tion and liver steatosis) and impaired circadian rhythms to exert their circadian control on behavioral and physiological (lengthened period of locomotor activity, body temperature processes. Both, the direct output of the SCN (prokineticin-2, and metabolic rate).20,21 transforming growth factor-a, g-aminobutyric acid and vaso- It is known that chromatin remodeling is a key process pressin) and the brain-mediated output (selective activation of evolved in CLOCK-regulated gene expression;23 in fact, the parasympathetic and sympathetic nerves, nocturnal secretion CLOCK protein has a significant activity as a histone of pineal melatonin) have been reported as mediators in the acetyltransferase facilitating gene transcription;24 this acti- effects of the SCN on the organism (Figure 1).5,12 vity is counteracted by Sirtuin-1, a histone deacetylase25,26 that is related to energy metabolism and aging.27 Recent evidence show that clock genes also act as sensors of the cellular metabolic status through changes in the redox The biological clock at a molecular level state.28 This dependence of the molecular clock on energy uptake could explain why alterations in metabolic cues, such Circadian clocks are composed of a set of proteins that gene- as restricted feeding or conflicting synchronizers, are able to rate self-sustained circadian oscillations through positive modulate the activity of the circadian timing system.29 It is

International Journal of Obesity Clock genes and obesity M Garaulet et al 1670

Cytoplasm Nucleous

CKIE CKIε CLOCK ?

BMAL RORE PPRE P 1 CRY1,2

CLOCK/NPAS2 PER 1-3 BMAL1

CRY1,2 CRY1,2

PER 1-3 PER 1-3

CLOCK/NPAS2 PPARA PPARα BMAL1 RORA RORα E-box

PPRE REV-ERBA REV-ERBα

CCGS

Cell

Figure 2 Organization of the mammalian circadian intracellular oscillator. The cellular oscillator is composed of a positive (CLOCK and BMAL1) and a negative (PER1–3 and CRY1, 2) limb. CLOCK–BMAL1 heterodimeres, through binding to E-box elements, drive the transcription of several genes: Cry1, Cry2, Per1, Per2, Per3, Rev-Erba, Rora and multiple CCGs. After dimerization PERs and CRYs undergo nuclear translocation inhibiting CLOCK–BMAL1-mediated transcription. Once the levels of PERs and CRYs fall, the negative repression is lifted and CLOCK–BMAL1 binds again to the E-box. A secondary stabilizing loop is established by the negative, REV-ERBa, and positive, RORa, effect on Bmal1 transcription through their activity on RORE. In addition, the PPARa, a CCG, induces Bmal1 and Rev-Erba transcription through its action on PPAR-response elements located in their respective promoters. The molecular circadian clock in linked to metabolism through several mechanisms. RORa and Rev-Erba regulates lipid metabolism and adipogenesis. BMAL1, brain- and muscle ANRT-like protein-1; CCG, Clock-controlled gene; CLOCK, Circadian Locomotor Output Cycles Kaput; CRY, cryptochrome circadian protein; PER, Period Circadian Protein; PPAR, peroxisome proliferator-activated receptor; REV-ERBa, erythroblastosis virus-a; RORa, retinoic acid receptor-related orphan receptor-a.

known that increased levels of the reduced cofactors NADH so-called clock-controlled genes (CCGs) or circadian output and NADPH consequence of energy supply by feeding or fat genes. A large percentage (3–20%) of the mammalian depots mobilization, induce an increase in the affinity of transcriptome shows circadian rhythms, including both CLOCK–BMAL1 and NPAS2–BMAL1 heterodimers for their direct CCGs and the downstream output of these CCGs3,35,36 E-box targets in vitro.28,30 In addition, NAD þ is required by (Figure 2). Sirtuin-1 as a co-substrate for their deacetylation activity, providing an additional link between energy cellular status and molecular clock.31 The balance between NAD þ /NADH is regulated by a key enzyme, the nicotinamide phos- What about the adipocyte? phoribosyl-transferase, also called visfatin. It is sensitive to cellular energy supply, being induced in hepatocytes of rats Although the clock mechanism is considered universal, after 48 h of fasting.32 It is noteworthy that its plasma values tissue-specific differences have been observed. Such differ- increase with obesity and type-2 diabetes.33,34 ences within cell types may provide important insight into In addition to these core clock genes, other genes of SCN the association between alterations in the circadian clock neurons, which are not components of the circadian and different diseases. Among peripheral tissues, AT has mechanisms, but whose expression is regulated by clock taken on a new importance given the current rise in obesity genes, oscillate with a periodicity close to 24 h. These are the epidemic.

International Journal of Obesity Clock genes and obesity M Garaulet et al 1671 Clock genes in AT from experimental animal models CD: pathological consequences Most molecular analyses of clock genes in AT have involved rodent models13,37–41 and evidence suggests that a fully Circadian disruption or CD could be defined as a relevant functional circadian clock mechanism exists within AT. In disturbance of the internal temporal order of physiological, isolated brown AT and white AT, clock genes were seen to biochemical and behavioral circadian rhythms. It is also a display an oscillatory profile, with anti-phase expression of breakdown of the normal phase relationship between the the positive (Bmal1, Clock) and negative (Cry, Per) arms, internal circadian rhythms and 24-h environmental cycles.54 respectively.13,37,39,41 When food access was temporally In our modern 24/7 society, CD may result from several restricted, the phase of clock genes shifted accordingly.13,42 conditions such as jet-lag, shift work, nocturnal light Several groups have evaluated clock genes in the AT of obese pollution or nocturnal leisure activities (Figure 3). mice4,37,43,44 and, while some observed that obesity had little In the recent years the effect of CD on human health has effect on the clock gene machinery,44 others found that it become an important issue. Multiple records link CD with attenuated and/or disrupted clock gene expression.37,43 This the increased risk of developing certain diseases and with an discrepancy may result from the murine strains used and the impairment of pre-existing pathologies: premature aging, fact that the mechanism of diet-induced obesity44 differs cancer, cardiovascular diseases, cognitive impairment and from that of obesity due to single mutations.37 mood disorders.55,56 Obesity and metabolic syndrome are In addition, some clock genes, especially Bmal1 and also closely related to CD. Rev-Erba, may play a part in adipocyte differentiation and lipogenesis.45This role of clock genes seems to be important Premature aging in the development of obesity. Similar to that observed in many physiological processes, the functioning of the circadian system changes with age. Phase advance, reduced amplitude, circadian fragmentation, Clock genes in human AT impaired ability to resynchronize after a time shift and Clock gene expression has been shown in human AT. internal desynchronization among different rhythms are the Furthermore, their expression has been associated with major characteristics of aged rhythms.57 However, less well different components of the MetS.46 It has also been shown known is the fact that CD has a direct role in inducing that clock genes can oscillate accurately and independently accelerated aging. For example, a 6-h shift of the light–dark of the SCN in human AT explants47 and that this intrinsic cycle every week induced a significant reduction in the oscillatory mechanism may participate in regulating the lifespan of aged mice.58 Longevity in hamsters decreased timing of other CCGs such as PPAR-g and glucocorticoid with circadian disruption and increased in older animals metabolism genes.47,48 Moreover, these circadian patterns given fetal SCN implants that restore higher amplitude differ between visceral and subcutaneous AT depots.47,48 circadian rhythms.59 Similarly, chronic reversal of the light– dark cycle decreases the median lifespan in cardiomyophatic hamsters.60 Therefore, it is generally believed that disrup- Circadian rhythmicity in AT metabolism tions in circadian rhythms lead to reduced life expectancy, AT metabolism has a strong correlation with circadian whereas their appropriate resetting leads to well-being and system.49,50 Over the course of a 24-h period, the adipocyte increased longevity. must reciprocally adjust the rates of triglyceride synthesis (lipogenesis) and storage with the rates of triglyceride breakdown (lipolysis). Diurnal variations in adipose meta- Cancer bolism are influenced by neurohumoral factors; for example The incidence and progression of some types of cancer, such by sympathetic activity (adrenaline). However, it has been as breast, endometrial, prostate, colorectal and melanoma, shown that such lipolysis persists ex vivo, showing that the can be promoted by CD.54,61 Both human epidemiological circadian clock within the adipocyte has a significant role by and experimental studies of animals have documented that a altering the sensitivity of the adipocyte to specific stimuli potential negative consequence of CD is cancer initiation (for example, insulin, adrenaline) and showing the intrinsic and development.62 nature of adipocyte diurnal variations.39 In addition, a relation between CD and cancer prognosis A number of adipocyte-specific factors show rhythmic has been observed in humans. In metastatic colorectal expression.5 Some examples are leptin, adipsin, resistin, cancer patients, marked rest–activity rhythms are associated adiponectin and visfatin, all of them showing circadian with a better quality of life, better response to chemotherapy rhythmicity. Glucocorticoids have also shown circadian and longer survival.63 rhythmicity in human AT.47 Adiponectin, a protector against MetS disturbances,51 shows both ultradian pulsatility and a diurnal variation.52 The daily pattern of this adipocytokine Cardiovascular diseases is out of phase with leptin, showing a significant decline at In people with normal blood pressure (BP) and uncompli- night and reaching a nadir in the early morning.53 cated essential hypertension, BP declines to its lowest levels

International Journal of Obesity Clock genes and obesity M Garaulet et al 1672

CAUSES CHRONODISRUPTION CONSEQUENCES

INPUTS Obesity -Weak Zeitgebers

-Conflicting Zeitgebers Metabolic syndrome -Zeitgeber shift

OSCILLATORS Cancer

-Uncoupling SCN oscillators

-Uncoupling SCN- Cardiovascular disease peripheral oscillators

-Clock gene alterations Mood disorders

OUTPUTS

-Melatonin Cognitive impairment -Cortisol

Figure 3 The causes and consequences of circadian disruption. CD is the result of a phase shift in the oscillation of the circadian (solid line) and activity-controlled physiological processes (dotted line). This circadian pathology can be induced by factors related to the following: Inputs: low contrast between day and night synchronizing signals (continuous light, frequent snacking, low physical exercisey); by Zeitgebers with different periods or unusual phasing (that is, light at night, nocturnal eating, nocturnal physical activityy) or by Zeitgeber shifts (that is, daylight-saving time, crossing time zones, shift worky). Oscillators: The uncoupling between the different oscillators inside the SCN caused by aging, the uncoupling between the central and peripheral oscillators or clock gene functional alterations result in circadian disruption. Outputs: Nocturnal melatonin suppression and loss of cortisol rhythmicity are also chronodisrupters. Many pathological states can be promoted or impaired as consequence of CD. CD, chronodisruption.

during night-time sleep, rises abruptly with morning Psychological diseases awakening and attains a maximum during diurnal activity. CD is also associated with an increased frequency of affective It has been shown that night-time BP is the best predictor of disorders such as seasonal affective disorder, major depres- stroke and myocardial infarction risk.64 Thus, hypertensive sion and insomnia.56 Finally, cognitive impairment asso- patients with a normal reduction in nocturnal BP (dipper) ciated with Alzheimer’s disease, vascular dementia or had a relative hazard of cardiovascular mortality similar to Parkinson’s disease can be aggravated by CD, as is observed that in non-dipper normotensives.65 It is noteworthy that when such sufferers spend too much time indoors or in a the non-dipper circadian pattern is more frequent among constant environment such as hospital wards or in intensive shift workers and elderly people. care units.66

Obesity and MetS What causes CD? As will be discussed in forthcoming sections, a well-known effect of CD on human health is the development of obesity CD can be induced by any impairment of the functioning of and MetS. Many epidemiological studies show that CD the inputs, of the 24-h oscillators or of the outputs of the induced by shift work, sleep deprivation or by shifting the circadian system (Figure 3). normal feeding time to night hours, is associated with high risk of developing obesity and many characteristics of metabolic syndrome.5 All the different metabolism-related Inputs activities of the circadian system, such as regulation of lipid Light–dark. Light–dark cycle is the main synchronizing and glucose metabolism, or insulin response, which are input to the central circadian pacemaker, the SCN. Light impaired by CD, may contribute to the pathophysiology of activation by blue-rich lights of the ganglion-containing obesity. melanopsin cells in the retina is sufficient for circadian

International Journal of Obesity Clock genes and obesity M Garaulet et al 1673 synchronization. However, circadian photoreception BMAL1 and premature aging. Bmal1-knockout mice, for decreases with normal aging as a result of age-related example, are arrhythmic and represent one of the best pupillary miosis and reduced crystalline lens transmission, models of premature aging. The mean lifespan of knockout particularly for blue light.66 Thus, people in their eighties animals was 37 weeks as compared with 120 weeks for wild- retain only 10% of the circadian photoreception of a 10-year- type animals.68 Other characteristics of this premature aging old child. In addition, contemporary artificial light sources model are sarcopenia, alteration of the percentage of do not provide more that 1% of the brightness of outdoor lymphocytes and impaired vision. natural light, with spectra shifted to longer wavelengths, which are less effective for SCN synchronization. Light PER2 and cancer. Although the mechanism through which deficiency, whether due to improper timing, suboptimal CD promotes cancer development is not well known, spectrum or insufficient intensity, may contribute to medical disruption of the core clockwork by mutations in Per2 conditions associated to CD.66 increases the susceptibility of mice to spontaneous and irradiation-induced tumors.69 This non-functional clock Feeding time. Feeding time is considered one of the most gene, Per2, was associated with changes in two genes, the important external synchronizers or Zeitgebers for peripheral proto-oncogene c-myc and the tumor repressor p53. Stimu- oscillators. Thus, unusual feeding times can also contribute lation of the proto-oncogene, together with repression of the to the effects of CD. For example, when nocturnal (charac- oncostatic p53 gene, could lead to genomic instability, terized by late awakening, omitting breakfast and late cellular proliferation and cancerogenesis. dinner) and diurnal (early awakening and early dinner) lifestyles were compared among healthy young people, CLOCK gene and obesity. CLOCK protein is a key protein plasma glucose and insulin response were found to be well involved in the synchronization of metabolic processes with regulated in the diurnal group, whereas a sustained hyper- the environment and in the control of the mammalian glycemia during the night and hypoglycemia in the morning energy balance. Circadian rhythms of both food intake and were observed in the case of the nocturnal lifestyle.67 In metabolic rate in Clock-mutant mice are abolished or addition, nocturnal leptin and melatonin were reduced with reduced. Clock-mutant mice are obese, show adipocyte the nocturnal lifestyle. Therefore, nocturnal lifestyle is likely hypertrophy, hepatic steatosis and alteration in the blood 4 to be one of the risk factors to the health of modern people, levels of leptin. including night eating syndrome, obesity and diabetes. However, significant controversy exists in these aspects. Kennaway et al.70 shown that Clock (Delta 19) mutation with arrhythmic Clock gene expression in the liver and skeletal Pacemakers muscle did not cause obesity and increased insulin sensitiv- Different rates of synchronization. As regards the pacemaker ity. While serum triglycerides and free fatty acids (FFAs) were 71 contribution to CD, classically, different rates of synchro- significantly lower in circadian Clock-mutant mice. The nization of biological variables have been suggested as the lack of reports for some other circadian mutants apart from cause of jet-lag and shift-work-induced CD. This may be the Clock and Bmal1 suggests that obesity and MetS are not a result of the different coupling strength between the SCN result from general disruption of rhythmicity and synchro- and the peripheral oscillators. For example, following a 6-h nization between different processes in tissues. Probably phase delay, the acrophase of adrenocorticotropic hormone some circadian proteins have, in addition to their role in the and cortisol rhythms take 7 days to resynchronize, whereas circadian system, specific non-circadian functions important the acrophase of the sleep–wake cycle resynchronizes in to the regulation of body fat. 3 days. Thus, for several days and because each organ shifts at a different rate, the organism is sub-optimally organized Outputs. The third element, which can cause CD, is the to efficiently accomplish its functions. alteration of the outputs of the circadian system. Impair- ment of the melatonin rhythm seems to be relatively well known. Melatonin is produced in response to a sympathetic Circadian clock alteration. In addition to this classical output of the SCN with a relay in the upper cervical hypothesis, CD can also be produced by alteration of the ganglion. Because melatonin is produced during the night core machinery of the molecular circadian clock. BMAL1, in all species studied, it is frequently called the ‘chemical PER2 and CLOCK, among others clock proteins, have darkness’. However, in addition to being controlled by the a specific role in an organism’s physiology in addition to pacemaker, light at night can induce an acute suppression of their role in the circadian molecular clock.55 A deficiency in melatonin secretion and its nocturnal plasma level shows a particular clock protein results in two conditions: disrup- progressive reduction with aging. Melatonin provides a tion of circadian rhythms and development of a primary potential connection between CD and the pathological pathology. Alterations in particular clock genes have been states induced by it.55,62 Melatonin is a well-known anti- associated with specific diseases, some examples of which are oxidant, which directly scavenges free radicals. In addition, mentioned below. the antioxidant activity of melatonin is connected with

International Journal of Obesity Clock genes and obesity M Garaulet et al 1674 induction of the expression/activity of major antioxidant increased exposure to bright light during the night, induce enzymes and with the improvement of the function of the brain to lose its ‘feeling’ for internal and external rhythms. electro-transport chain in the mitochondria.72,73 The lack of day–night environmental contrast may lead to Besides its antiaging effects, melatonin is considered as an CD and metabolic disturbances, including obesity. Conver- anticarcinogenic and oncostatic substance, particularly sely, studies performed using experimental models have against, breast, colorectal, prostate and melanoma cancers. shown that developing obesity and diabetes itself disrupts Melatonin has been also related to obesity alterations, acting the molecular clock system.37 Both, as a cause or as a as a protective hormone. It reduces BP and improves glucose consequence, CD is closely linked to obesity. Some of the metabolism.74 following theories have been put forward:

The autonomic nervous system Obesity as a chronobiological illness: As the brain uses the autonomic nervous system to imple- epidemiological evidence ment internal rhythmicity, an unbalanced arrhythmic autonomic nervous system has been proposed as a major There is epidemiological evidence of a relationship between cause of obesity. obesity and chronobiological aspects. One of the most One of the crucial components of metabolic syndrome in interesting findings is that shift work is an independent risk obesity is visceral fat. Many studies show that CD is more 75 factor in the development of obesity. Industrialization has frequent among obese subjects with a central distribution of given rise to the widespread adoption of 24-h continuous fat than in those with a gluteofemoral pattern. Furthermore, operations in a number of industries. This has resulted in an the chronotype of the subject has been shown to be related increase in the proportion of the population routinely to obesity, with ‘evening type’ subjects being more likely to engaged in shift work, reaching 420% of the industrialized display central obesity than morning types. world. Epidemiological studies show that shift work is The high specialization of autonomic nervous system associated with obesity, hypertriglyceridemia, low high- neurons allows them to project either to the intra-abdominal density lipoprotein, abdominal obesity, diabetes and cardio- or subcutaneous body compartment, and this depot-specific 76 vascular disease. Furthermore, increased glucose, insulin effect could also vary depending on the time of the day.81 As and triglyceride postprandial metabolic response is observed a consequence, intra-abdominal organs, such as visceral fat, in shift workers with disrupted circadian rhythmicity of the liver and pancreas, receive input from the same neurons and, 77 melatonin profile. therefore, the same stimulus will increase insulin sensitivity Together with shift work, a similar situation underlies the at the same time in the three locations. It has been sleep ‘disorders’ of jet lag, which is not in itself a disease but hypothesized that as meals consumed in the evening are rather reflects a normal function of circadian timing in the known to lead to enhanced insulin secretion, a shift of context of extraordinary demands on sleep–wake schedul- feeding toward the end of the day will result in a more ing. Both jet lag and shift work insomnia are related to efficient uptake of glucose in visceral fat and will result in obesity and represent important social problems, which easier accumulation of intra-abdominal fat. However, further 78 deserve public health and medical attention. studies should confirm this hypothesis considering that Interesting results have arisen from studies relating sleep insulin resistance is also increased during night hours. duration and metabolic risk. The time we sleep has declined by 1.5 h over the past century, accompanied by an important increase in obesity. Moreover, a third of adults sleep less than The pineal–hypothalamic–adipocyte pathway six hours a night.79 Clinical studies show that healthy In the same way, but related to the pineal–hypothalamic– 82 subjects restricted to 4 h of sleep for six consecutive nights adipocyte pathway, Scott and Grant proposed a similar showed impaired glucose tolerance and reduced insulin hypothesis on the basis of the fact that hibernators are a responsiveness after a glucose challenge.80 These metabolic good model of human obesity. In these animals the patterns changes are rather concerning children, in whom short sleep of fat acquisition follow endogenous circannual rhythms duration, affected by factors such as the day of the week, entrained by ambient light. In preparation for winter, during season and having younger siblings, has been described as an the period of fat storage, there is an insulin resistance state independent risk factor for obesity.80 during which weight almost doubles through storage of vast depots of fat. Thus, it can be hypothesized that modern westernized man are always getting ready for a food deprivation state (winter) that never comes. Circadian disruption and obesity: different theories

The precise mechanisms linking obesity to CD are not well Internal environment challenges within the organism known. It has been hypothesized that current habits, such as Most hypotheses to explain the relationship between obesity high snacking frequency, a reduction in total daily sleep and and chronobiology point to internal desynchronization

International Journal of Obesity Clock genes and obesity M Garaulet et al 1675 between the different circadian rhythms involved in meta- rhythms of peptides involved in food intake control, bolism as a key factor. Some examples are the following: especially of leptin, could be involved in the imbalance between energy intake and expenditure. Conversely, high Food intake signals. Excess energy intake produces a pro- energy intake, constant snacking or alteration in meal times gressive derangement in temporal communication between could be affecting this internal circadian machinery. different food intake signals by modifying the strength, Sleep alterations are also related to food intake. For duration and frequency in their rhythmicities. An example is example, several studies have shown that short sleepers have the well-known reciprocal circadian and ultradian rhythmi- significantly reduced circulating levels of the anorectic cities in anorexigenic leptin and orexigenic ghrelin, which hormone leptin and increased levels of ghrelin, an orexi- 90 encode a corresponding release pattern of appetite stimulat- genic stomach-derived hormone, suggesting that sleep ing neuropeptide-Y (NPY) in the hypothalamus. deprivation may affect the peripheral regulators of hunger. With obesity, hyperleptinemia leads to a central leptin Furthermore, leptin-deficient mice show impaired sleep and 91 insufficiency and breakdown in this coordinated rhythmic altered diurnal rhythmicity, whereas ghrelin has been interplay. Similar theories have been proposed for histamine shown to increase non-rapid eye movement sleep in both 92,93 and H1 receptors, apoprotein A-IV and other peptides highly rodents and humans. implicated in food intake regulation. The circadian pacemaker and cytokine systems seem to overlap with respect to their influence on energy intake patterns and sleep–wake regulation.94 Indeed, there is LPL and meal times. On the basis of its ability to cleave growing evidence that interleukin-6 and several other circulating triglycerides, lipoproteinlipase (LPL) was origin- proinflammatory cytokines are ‘sleep factors’.95 In addition, ally identified as a clearing factor as it releases FFAs and it has been shown that these cytokines also influence energy facilitates their uptake across the cell’s plasma membrane.83 intake by enhancing insulin and leptin sensitivity.96 How- Under physiological conditions FFAs are transported to the ever, any possible role they might have in coordinating adipocytes where they are stored for subsequent use. Gimble sleep/wakefulness with food-motivated behavior is still et al. recently described that ‘If AT expressed LPL continu- unclear. There is increasing evidence that the neurophysio- ously, the time that an individual ingested a fatty meal logical and metabolic mechanisms responsible for control- would be irrelevant. However, LPL mRNA and protein levels ling food-seeking behavior and sleep/wakefulness are show circadian oscillations, and, as long as LPL peaks at meal coordinated. Indeed, hunger and vigilance are paired during times, the FFAs will be properly stored within the adipocytes. daylight hours, whereas satiety and sleep are paired during By contrast, if meal times do not coincide with LPL peaks, the dark.94 Recently we have reported that CLOCK gene the organism may store circulating FFAs in ectopic tissues, polymorphisms are significantly associated with food intake resulting in lipotoxicity and, as a consequence, hepatic, and that the cytokine system, particularly interleukin-6, has muscular or pancreatic comorbidities’.83 a significant role in this connection.97 Most of the hypotheses proposed to date point to the Other neuromodulators involved in feeding and alertness, same: perhaps obesity is on the rise not only because we including histaminergic and serotoninergic transmitters, consume too much food, but also because we eat it at the may also jointly affect alertness, circadian rhythmicity and wrong times. In this line an interesting study has shown that metabolism. nocturnal mice fed a high-fat diet only during the 12-h light phase gain significantly more weight than mice fed only 84 during the 12-h dark phase. A physiological explanation of this relationship Experimental studies of sleep deprivation98 suggest that a number of causal pathways link short sleep duration and Energy intake and expenditure: the relevance obesity (Figure 4). One mechanism by which sleep depriva- of sleep tion might contribute to weight gain is by increasing caloric intake. Experiments in animals have consistently Classical lesion studies performed more than 50 years ago found that total sleep deprivation produces hyperphagia,99 showed that distinct regions of the hypothalamus control whereas partial sleep deprivation experiments in humans hunger and satiety. A relevant aspect in those studies was the suggest a similar effect especially as regards high-fat and cloning of leptin, which decreases food intake and body high-carbohydrate foods. Such changes in sleep patterns are weight. Leptin decreases hunger and increases energy accompanied by increased serum ghrelin levels, increased expenditure through stimulation of the melanocortin evening concentrations of cortisol and decreased levels of system.85–89 The fact that leptin is expressed in a circadian leptin. Alternatively, in an environment where food is manner and fluctuates in response to fasting and feeding89 readily available, reduced sleep may simply represent an reinforces the importance of chronobiology for controlling increased opportunity to eat. Chronic partial sleep depriva- food intake. An important implication arising from these tion also clearly leads to a feeling of fatigue,100 which could results is that ‘an internal desynchronization of the circadian lead to a reduction in physical activity. Indeed, studies of

International Journal of Obesity Clock genes and obesity M Garaulet et al 1676 ↑ Orexin ↓ Leptin ↑ Ghrelin ↑ Appetite ↑ Food intake ↑ Time and opportunity to eat Obesity

Sleep Altered deprivation thermoregulation ↓ Energy expenditure ↑ Fatigue

↑ Evening cortisol ↑ Nighttime GH ↑ Insulin Resistance Metabolic ↓ Glucose tolerance syndrome ↑ Sympathetic Activity

Figure 4 A schematic representation of the putative pathways leading from sleep loss to obesity and the MetS. MetS, metabolic syndrome.

children have found short sleep duration to be associated It is of note that of the genes that show circadian with increased television viewing and reduced participation rhythmicity in mouse liver,104 2.5% also showed altered in organized sports.90,101 Sleep loss may also affect energy expression in rat liver when 40% of the energy in the diet was expenditure through thermoregulation, and studies of acute fat (lard, olive oil, fish oil or coconut). It is assumed that sleep deprivation in humans have pointed to a reduction in habitual high-fat meals perturb the circadian rhythm of these the core body temperature.102 genes, either by flattening the rhythm or by magnifying it. Further studies are necessary to confirm the particular roles of the different types of fat in the circadian system. Fat intake and obesity Olive oil as a source of monounsaturated fatty acids may affect circadian rhythmicity in a different way to saturated One of the most important aspects of the interaction fat. In this line our results in a US white population support between chronobiology and obesity is the effect that high- the notion that the genetic effects of CLOCK on the insulin fat feeding has on both. Epidemiological studies have shown resistance and obesity phenotypes could be modulated by that fat intake is related to CLOCK gene polymorphisms.97 dietary intake of monounsaturated fatty acids. Indeed, the However, data are still contradictory. Although experimental protective effect of the minor allele on insulin sensitivity was studies have shown alterations in clock gene expression in only present when monounsaturated fatty acid intake was AT and liver, they failed to show any attenuation of clock higher than the mean value (13.2% of energy).105 gene expression in the hypothalamus.44 This suggests that high-fat feeding leads to obesity and affects the molecular clock function in peripheral tissues but not in the central Psychological aspects in obesity and chronobiology nervous system. Such changes are associated with altered diurnal profiles of leptin, glucose, insulin, FFAs and cortico- Obesity is highly related to emotional disorders. It has long sterone, as well as impaired regulation of nuclear receptors, been known that the frequency of overweight and obese suggesting that this disruption of peripheral clocks directly people is higher among depressed and bipolar patients than alters the metabolic function.43 Recent experimental models in the general population. Marked alteration of body weight have shown that high-fat feeding, particularly high intake of (and appetite) is one of the most frequent symptoms of saturated fat, modifies circadian synchronization to light major depressive episodes. In the case of obesity, unipolar and leads to metabolic abnormalities, which mimic human or bipolar depression is frequently observable (20–45%).106 MetS, including obesity and insulin resistance.103 In depressed patients, obesity is related not only to

International Journal of Obesity Clock genes and obesity M Garaulet et al 1677 depressive episodes but also to lifestyle factors, to comorbid preference in healthy humans and that cause sleep phase bulimia and probably to genetic–biological factors.106 As delay and insomnia in patients with different physiological melancholic depression is known to be associated with illnesses.113–115 These alterations could be related to obesity, elevated plasma cortisol levels, depressed patients are prone but also to weight loss. In this line, we have previously shown to excess visceral fat storage, with the subsequent risk of that most alterations involved in the obstacles of weight loss developing associated metabolic disturbances.107 At the are related to the subject’s eating behavior and psychological same time, according to certain studies, circadian symptoms characteristics, and that these obstacles are directly related to the of depression give rise to metabolic processes in the body, difficulty in losing weight.116 Furthermore, previous works have which eventually led to obesity and insulin resistance. related rs1801260 (3111T4C) with eating disorders and Night eating syndrome, defined as ingestion of 425% of obesity.117,118 This idea seems to be supported by findings in daily calories after dinner and nocturnal awakenings with animals, which suggest a direct involvement of the CLOCK gene ingestion, is also more frequent among obese subjects. in the regulation of body weight, as homozygous Clock-mutant Indeed, such a disorder occurs at an incidence of 1.5% in micedevelopedobesity,hyperphagiaandalsosufferedfrom the general public but at 9% in the obese and 425% in the changes in eating behavior, sleeping and mood.4 Our own group morbidly obese populations.108,109 has shown that CLOCK gene polymorphisms are implicated in There is increasing evidence that a dysfunctional circadian the effectiveness of a behavioral treatment of weight loss.119 system could be a primary cause of altered emotional In this line, a missense mutation in the human PER2 behavior. Particularly in the case of night eating syndrome, gene has previously been linked to psycho-behavioral it has been reported that subjects show a 1- to 3-h phase shift factors such as diurnal preference,120 advanced sleep phase in the acrophase of their circulating leptin and cortico- syndrome,121 seasonal variations in mood and behavior and sterone levels.108,109 winter depression.122 Very recently, Yang et al.123 have Experimental studies show that Clock homozygous mutants shown that Per2À/À mice showed a feeding abnormality that have a spectrum of behavioral abnormalities, including low resembles that of the night eating syndrome, which has the anxiety, mania and hyperactivity.110,111 The hypothesis that combined features of a circadian rhythm disorder and an certain genetic susceptibility factors are shared across the eating disorder.123 Most of the psychological alterations psychosis spectrum112 has led several groups to investigate associated with PER2, such as depression or seasonal affective clock gene associations in a number of mood disorders and disorder, were often accompanied by alterations in sleep psychoses. The most evident example is Seasonal Affective patterns and eating behavior, including overeating and Disease, which was associated with single-nucleotide poly- craving for carbohydrates.124 Such results suggest that a morphism (SNP) variants in PER2 and BMAL1. study of different PER2 polymorphisms could be of interest Different research groups are currently investigating in obesity. Furthermore, our group has shown that PER2 is associations between clock genes and bipolar disorders, implicated in attrition in weight loss treatment and may schizoaffective disorder, schizophrenia and autism. Conflicts modulate eating behavior-related phenotypes.125 between internal biological clocks and environmental (solar) and social clocks may well be evident in individuals with Seasonal Affective Disease and other emotional alterations. Genetic aspects Particularly in the case of the CLOCK polymorphism rs1801260 (3111T4C),ithasbeenshownthatitisrelatedto The circadian biology of AT has been evaluated in multiple clinical features of mood disorders that influence diurnal gene-deficient or mutated rodent models (Table 1). Defects

Table 1 Gene-deficient or mutated rodent models to evaluate the circadian biology of AT

Genea Adipose biology Circadian biology

Agouti Obesity Bmal1À/À Reduced adipose volume Clock (mutant) Obesity Arrhythmic HSF1À/À Reduced adipose volume Arrhythmic LeptinÀ/À Obesity Attenuated sleep–wake cycle, reduced body temperature Melanocortin receptor-3À/À Obesity Disrupted clock genes expression profile, altered food anticipatory activity Non-obese diabetic rat Non-obese No clock gene attenuation with exception of Per2 in adipose tissue NocturninÀ/À Resistance to diet-induced obesity and hepatic steatosis PGC1bÀ/À Defective triglyceride storage, defective BAT fatty acid Decreased physical activity oxidation and thermogenesis PPARaÀ/À Defective BAT fatty acid oxidation and thermogenesis Altered food entrainment of clock genes in brown adipose tissue and regulation of downstream target genes

Abbreviations: AT, adipose tissue; BAT, brown adipose tissue; Bmal, brain- and muscle ANRT-like protein. Table adapted from Gimble et al.59 aUnless otherwise stated, all models are murine.

International Journal of Obesity Clock genes and obesity M Garaulet et al 1678 in some clock genes lead to irregular circadian and adipose easily to desynchronization from the central pacemaker and biology. AT volume is reduced in Bmal-deficient mice but its determination interferes with the sleep–wake cycle. Only increased in murine Clock mutants. It was the study by Turek with the use of such objective techniques can the effect of et al.4 that provided the evidence of a molecular interaction CD and chronoenhancement on obesity be accurately between clock genes and obesity. This study showed that assessed. mice with a disrupted Clock gene were prone to developing To establish a chronobiological-based therapy for obesity, a phenotype resembling MetS. three levels of potential therapeutic interventions, on Adipokine and receptor gene deficiencies associated with the basis of the functional organization of the circadian obesity (leptin, melacortin receptor) lead to abnormal system, can be considered: the inputs to the pacemakers, the circadian rhythms. Furthermore, clock disruption in leptin- pacemakers themselves and the output from the pacemakers deficient mice (ob/ob) led to greater weight gain, increased (Figure 5). triglyceride, cholesterol and adipocyte hypertrophy than leptin deficiency alone.45 Consistent with these findings, genetic polymorphisms in Inputs human clock genes have been associated with increased The adequate functioning of the circadian system, which in incidence of obesity in epidemiological studies. Different turn is highly important for health, is dependent on authors are currently investigating the role of clock gene regularly timed exposure to Zeitgeber stimuli. variants and their predicted haplotypes in human obesity Although no data about the influence of regular light and MetS alterations.99,105,117,119,126,127 From studies of our exposure on obesity have been published, several papers own group and others we deduce that CLOCK gene SNPs show that increase in bright light exposure and its regularity rs3749474, rs4580704 and rs1801260 (3111T4C) are improves function in a number of clinical conditions closely particularly interesting as they seem to be associated with related to obesity, including bulimia,131 anorexia,131 insom- body mass index (BMI), energy intake and different obesity- nia and depression.56 related variables.97,119 The PER2 SNPs rs2304672C4G and Furthermore, sleep loss and sleep instability have also been rs4663302C4T are also remarkable because have been related to impaired glucose and lipid metabolism, and to related to abdominal obesity.125 MetS development. To achieve optimal sleep–wake rhythm regulation, adherence to a regular sleep schedule and, particularly, to a regular getting up time is fundamental. The regularization of sleep–wake schedules for 4 weeks in Chronobiology in the treatment of obesity young adults led to a significant improvement in daytime alertness and nocturnal sleep efficiency.132 To date, most studies have focused on the pathophysiologi- cal consequences of CD. However, very few papers have been Feeding rhythms. Feeding rhythms have a strong influence published about how chronobiology can be used in the on peripheral oscillator synchronization, and feeding regu- effective treatment of obesity and MetS. As CD can be larity and the timing of nutrient intake should be taken into considered a risk factor in the development of obesity, we consideration. To date, no studies of humans have been hypothesize that the appropriate resetting of the circadian published evaluating the influence of a sustained more system (chronoenhancement) may lead to reduced inci- regular 24-h pattern of food intake on the circadian system. dence of obesity. However, animal studies show that regular restricted feeding To study the effect of CD and chronoenhancement on availability improves the synchronization of the circadian morbidity and mortality with a high degree of accuracy, it system.6 It is likely that regular feeding times also affect the is important to quantitatively characterize the human synchronization of the circadian timing system through circadian system status. To this end, several tests and diaries their effect on cortisol and body temperature changes due to have been extensively used. However, because of the poor diet-induced thermogenesis. accuracy of such procedures, some circadian marker Better known is the effect of the timing of nutrient intake rhythms, which are under the control the SCN, have become on metabolism. For example, in young healthy people, the widely used. For example, core temperature actimetry, results of the oral glucose tolerance test worsened when salivary melatonin and cortisol rhythm are the most glucose was ingested during the evening as compared with frequently used methods. Recently, we proposed the use of that in the morning. Circadian variation in glucose tolerance wrist skin temperature, recorded by means of a wireless data in rats was related with a reduction in insulin sensitivity.133 logger, as a reliable circadian index for assessing the human Furthermore, when the total daily caloric intake was ingested circadian system status.128 Molecular markers, consisting of as one meal (dinner), an increase in body weight as serial determination of clock genes in peripheral blood cells compared with the same meal ingested in the morning was and in oral mucose cells,129,130 also have been proposed to observed.134 evaluate human circadian system; however, both tissues Besides light exposure, sleep–wake pattern and feeding contain peripheral clocks and, thus, they can be subjected time, physical exercise is a relevant Zeitgeber for the

International Journal of Obesity Clock genes and obesity M Garaulet et al 1679 OBESITY TREATMENT

INPUTS OUTPUTS

- Regular light exposure - Pharmacological: Melatonin - Optimal sleep regulation - Regulation of cortisol - Regular feeding time rhythms - Regular exercise

PACEMAKERS

- Nutrigenetic (CLOCK, PER2 polymorphisms)

- Individual chronobiologic characterization

Figure 5 Different strategies in the treatment of obesity from a chronobiological perspective.

circadian system and is a key factor in the prevention and diagnosis of obesity and may predict the outcome of body treatment of obesity. Epidemiological studies indicate that weight reduction strategies on the basis of low-energy diets. prolonged and regular exercise is associated with better In this sense our research group has shown that the CLOCK nocturnal sleep and lower daytime tiredness.134 Prolonged rs1801260 SNP may predict the outcome of body weight regular exercise in elderly people also counteracts the age- reduction strategies on the basis of low-energy diets.119 Such related fragmentation of rest–activity rhythms,135 and in new strategies could represent a step toward personalized sedentary elderly subjects, exercise improves performance health care and nutrition on the basis of a combination of in executive control tasks136 and prefrontal activation.137 genotyping and chronobiological characterization. However, the objective effect of exercise through the circadian system may develop only slowly. For example in a study performed in rats concerning the effect of exercise Outputs on obesity induced by periodical phase shifts, the significant As regards pacemaker outputs, melatonin and cortisol effect of regular exercise on body weight control appeared rhythms have been relatively well studied and are a known only after 3 months.134 link between CD and metabolic disturbances. Much evidence supports the ability of melatonin to resynchronize the sleep–wake rhythm in blind people, as well in jet-lag and Pacemakers shift-work. Melatonin is an example of a chemical output Contrasting with the relatively known therapeutic effects of from the SCN acting simultaneously as an input to the SCN. input manipulation, direct interventions on pacemakers are Melatonin induces sleep and therefore enhances circadian difficult to perform and scarce. Knowledge is looking to robustness by binding to membrane receptors MT1 and MT2 achieve pharmacological success in the effective manipula- in the SCN. Animal studies pointed to the positive health tion of SCN without causing adverse effects on this and other consequences of prolonged regular melatonin administra- systems. Nevertheless, on the basis of recent epidemiological tion.138 Most studies indicate that regular melatonin treat- studies showing a link between some clock gene polymor- ment increases the lifespan of animals, probably by means phisms and obesity, a new chronobiology treatment strategy of its antioxidant effects and through induction of an could be proposed in the near future. enhanced circadian robustness. New, recently synthesized The success of obesity therapy is, at least in part, depen- melatonin agonists such as Ramelteon and Agomelatine dant on the genetic background of a patient.138 The identi- can improve circadian rhythmicity through promotion of fication of clock gene polymorphisms could be useful in the sleep and improving depressive states, respectively, without

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