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Circadian Rhythm Sleep-Wake Disorders Review Article Address correspondence to Dr Milena Pavlova, Brigham Circadian Rhythm and Women’s Faulkner Hospital, 1153 Centre St, STE 4H, Boston, MA 02130, Sleep-Wake Disorders [email protected]. Relationship Disclosure: Milena Pavlova, MD, FAASM Dr Pavlova serves as associate editor of the Sleep and Chronobiology section of Frontiers in Neurology and ABSTRACT receives research/grant support from Biomobie Inc Purpose of Review: The endogenous circadian rhythms are one of the cardinal and Lundbeck. processes that control sleep. They are self-sustaining biological rhythms with a Unlabeled Use of periodicity of approximately 24 hours that may be entrained by external zeitgebers Products/Investigational (German for time givers), such as light, exercise, and meal times. This article discusses Use Disclosure: Dr Pavlova reports no the physiology of the circadian rhythms, their relationship to neurologic disease, and disclosure. the presentation and treatment of circadian rhythm sleep-wake disorders. * 2017 American Academy Recent Findings: Classic examples of circadian rhythms include cortisol and of Neurology. melatonin secretion, body temperature, and urine volume. More recently, the impact of circadian rhythm on several neurologic disorders has been investigated, such as the timing of occurrence of epileptic seizures as well as neurobehavioral functioning in dementia. Further updates include a more in-depth understanding of the symptoms, consequences, and treatment of circadian sleep-wake disorders, which may occur because of extrinsic misalignment with clock time or because of intrinsic dysfunction of the brain. An example of extrinsic misalignment occurs with jet lag during transmeridian travel or with intrinsic circadian rhythm sleep-wake disorders such as advanced or delayed sleep-wake phase disorders. In advanced sleep-wake phase disorder, which is most common in elderly individuals, sleep onset and morning arousal are undesirably early, leading to impaired evening function with excessive sleepiness and sleep-maintenance insomnia with early morning awakening. By contrast, delayed sleep-wake phase disorder is characterized by an inability to initiate sleep before the early morning hours, with subsequent delayed rise time, leading to clinical symptoms of severe sleep-onset insomnia coupled with excessive daytime sleepiness in the morning hours, as patients are unable to ‘‘sleep in’’ to attain sufficient sleep quantity. Irregular sleep-wake rhythm disorder is misentrainment with patches of brief sleep and wakefulness spread throughout the day, leading to unstable sleep and waking behavioral patterns and an entirely idiosyncratic sleep-wake schedule. Summary: Familiarity with these major circadian rhythm sleep-wake disorder phenotypes and their overlap with other neurologic disorders is essential for the neurologist so that clinicians may intervene and improve patient functioning and quality of life. Continuum (Minneap Minn) 2017;23(4):1051–1063. INTRODUCTION The term circadian, when applied to A natural, endogenous fluctuation is physiologic or pathophysiologic varia- observed in a variety of physiologic tions, implies that these rhythms are functions, the most classic example of endogenous and would continue to which is probably the menstrual cycle. occur in a similar pattern of approxi- The biological rhythms that have a mately 24-hour temporal periodicity in period of approximately 24 hours the absence of exogenous factors. are referred to as circadian rhythms. Naturally, a tachycardia in response Continuum (Minneap Minn) 2017;23(4):1051–1063 ContinuumJournal.com 1051 Copyright © American Academy of Neurology. Unauthorized reproduction of this article is prohibited. Circadian Rhythm Sleep-Wake Disorders KEY POINTS AM h Circadian rhythms are to a 6:00 alarm would not be ‘‘day’’), a protocol that was used to endogenous rhythms considered a circadian function, al- determine the ‘‘forbidden zone for 3 that control various though it occurs within a 24-hour sleep.’’ Some protocols are ultralong, 4 physiologic functions period. However, it is often difficult such as a 42-hour day. In optimal and are one of the to distinguish whether a 24-hour fluc- conditions, circadian rhythms are main factors that tuation in a specific physiologic func- aligned with each other and get re- control sleep. tion is the result of an endogenous aligned (entrained) with the social and h Circadian rhythms have process or of environmental stimuli. natural environment. When the en- been observed to affect Furthermore, most physiologic func- dogenous propensity to sleep is a wide variety of tions are also affected by the states misaligned with the needs of the endocrine functions, of sleep and wakefulness, and since social function, this can lead to inap- gastric acid secretion, sleep typically occurs at night, a propriate sleep times and difficulty motor activity pattern, nighttime process may be due to sleeping (circadian rhythm sleep-wake breathing, blood either sleep or circadian phase. Some disorders). pressure, as well as both of the early studies on circadian normal and abnormal rhythms were conducted in World PHYSIOLOGY OF CIRCADIAN central nervous RHYTHMS system activity. War II bunkers or deep caves devoid of natural light in an attempt to isolate Circadian rhythms have been observed external influences. Later, two major to affect a wide variety of endocrine types of experimental protocols were functions, gastric acid secretion, motor established to study circadian rhythms.1 activity pattern,5,6 breathing,7 blood The first protocol is a constant routine, pressure,8 as well as both normal in which the individual is kept in dim and abnormal central nervous system light, in one near-constant position, activity.9Y13 In optimal physiologic con- and continuously awake (to eliminate ditions, the individually controlled bio- the effect of sleep-wake state) for 24 to logical rhythms function in synchrony 48 hours. This type of protocol has to allow optimal performance.5 Circa- been used to establish the primarily dian rhythms would continue to occur circadian nature of some major func- with a near 24-hour rhythm even in the tions, such as cortisol secretion, body absence of external time cues, al- temperature, urine volume, and mela- though the precision of and adaptation tonin, which have later been used as to the current environment (entrain- markers of circadian phase.2 ment) is ensured through external The second protocol is forced zeitgeber (German for time giver) desynchrony, in which the timing of environmental influences, including the sleep period is changed, so that ambient light (the most powerful envi- sleep occurs at times that are different ronmental time cue), activity and exer- from the individual’s typical sleep cise, and meal times. time. The length of the ‘‘day’’ in forced desynchrony is either shorter Role in Sleep-Wake or longer than 24 hours, while the Maintenance proportion of scheduled wakefulness The current model to explain sleep is and sleep remains 2:1 to allow obser- controlledbytwomainprocesses: vance of sleep in all circadian phases. homeostatic and circadian. The ho- The duration of the episodes varies meostatic process is a longitudinal with the nature of the function stud- day-long linear process that increases ied. Some protocols are ultrashort, in strength with the duration of wake- with 7 minutes allowed for sleep and fulness and is likely to be mediated by 14 minutes of wakefulness (21-minute adenosine accumulation through the 1052 ContinuumJournal.com August 2017 Copyright © American Academy of Neurology. Unauthorized reproduction of this article is prohibited. period of daytime activity, favoring an increased propensity toward sleep with cumulative wakefulness.14 The circa- dian process is primarily an endoge- nous biological rhythm, which increases in strength with the duration of the biological day, opposing and balancing the effect of the homeostatic drive, thus facilitating continuous wakefulness throughout the day.5 The same circa- dian process enables continuous sleep during the main nocturnal sleep period. While the function of circadian rhythms is endogenous and preserved in the absence of any external cues, the timing of the biological day, as noted above, is regulated by multiple exogenous factors FIGURE 7-1 Effect of light. The effect of ambient light is transmitted through the retinohypothalamic such as light, activity and physical pathway to the suprachiasmatic nucleus exercise, and meal times. (SCN) of the hypothalamus, which, in turn, affects the circadian rhythms of the rest of the body. The pineal gland receives input from the cervical sympathetic chain to Effect of Light: secrete melatonin, which, in turn, diminishes the alerting Retinohypothalamic signal from the circadian system. Pathway The effect of circadian rhythm on sleep is regulated by the suprachiasmatic a phase-advancing effect (Figure 7-2). KEY POINT nucleus of the hypothalamus, which The light spectrum,15,16 as well as h While the function of receives input via the direct retinohypo- timing and length of light exposure,17 circadian rhythms is thalamic pathway. Regular entrainment alter the timing of the biological day. In endogenous and of the organism with the external the absence of light, circadian rhythms preserved in the absence of any external environment is facilitated by hypotha- do not adequately entrain to environ- 18 cues, the timing of the lamic synchronization of endogenous mental cues. Diseases that affect
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