Health Consequences of Shift Work and Implications for Structural Design

REVIEW Health consequences of shift work and implications for structural design

MG Figueiro1 and RD White2

The objective of the study was to perform a literature review on the health consequences of working rotating shifts and implications for structural design. A literature search was performed in June 2012 and a selection of the most relevant peer-review articles was included in the present review. Shift workers are more likely to suffer from a circadian sleep disorder characterized by sleepiness and insomnia. Shift work is associated with decreased productivity, impaired safety, diminished quality of life and adverse effects on health. Circadian disruption resulting from rotating shift work has also been associated with increased risk for metabolic syndrome, diabetes, cardiovascular disease and cancer. This article summarizes the known health effects of shift work and discusses how light can be used as a countermeasure to minimize circadian disruption at night while maintaining alertness. In the context of the lighted environment, implications for the design of newborn intensive care units are also discussed.

Journal of Perinatology (2013) 33, S17–S23; doi:10.1038/jp.2013.7 Keywords: shift work; circadian system; melatonin; circadian rhythms; cancer; newborn intensive care unit design

INTRODUCTION Earth by developing circadian rhythms (circa ¼ about; die ¼ day), In the United States and Europe, 15 to 20% of all full-time wage which are rhythms that repeat approximately every 24 h. and salary workers work alternative shifts.1 Evening shifts, which Circadian rhythms are generated endogenously and are aligned fall between 1400 hours and midnight, are relatively common, with the environment by exogenous factors. It is widely known accounting for 4.6% of work. Shifts that include night work, such that in mammals, circadian rhythms are regulated by an internal as night shifts and rotating shifts, are most prevalent, however, biological clock located in the suprachiasmatic nuclei (SCN) of the 2 and account for 6.4% of all full-time wage and salary workers.1 hypothalamus of the brain. The SCN are self-sustaining oscillators Shift work, in particular that which includes night work, requires that maintain their daily activities for weeks when isolated and that the worker invert his/her activity-rest cycle. As a result, shift cultured. The SCN in humans have a natural period that is slightly workers are more likely to suffer from shift work disorder, a greater than 24 h. Environmental cues can reset and synchronize circadian sleep disorder characterized by sleepiness and/or the SCN daily, ensuring that the organism’s behavioral and insomnia. Shift work disorder is associated with decreased physiological rhythms are in synchrony with the daily rhythms in productivity, impaired safety, diminished quality of life and its environment. The light/dark cycle is the main synchronizer of 2 adverse effects on health. Moreover, circadian disruption resulting the SCN to the solar day. Light/dark information is received by from rotating shift work has also been associated with increased the eye and reaches the SCN via the retinohypothalamic tract. risk for metabolic syndrome, diabetes, cardiovascular disease and Circadian oscillations in most human cells are a result of a small cancer. group of clock genes inside the cell nuclei creating interlocked The present review paper summarizes the recent scientiﬁc transcriptional and post-translational feedback loops. These literature on the health effects of circadian disruption cellular oscillators are synchronized to the biological clock located associated with working rotating shifts and discusses some non- in the SCN. Neural and hormonal stimuli transmit time information pharmacological countermeasure options. Understanding these from the master clock to these peripheral clocks. To accomplish effects makes it possible to design lighting solutions that will aid this, the SCN communicate with neuronal targets such as in diminishing the negative effects of shift work on many types of endocrine neurons, autonomic neurons of the paraventricular workers, including nurses and doctors who work in various nucleus of the hypothalamus and the sub-paraventricular zone in hospital settings. the hypothalamus. The SCN control the release of melatonin through the autonomic nervous system of the paraventricular nucleus. Melatonin is a hormone produced by the pineal gland at night (onset occurs in the early evening, B2 h before sleeping) CIRCADIAN RHYTHMS and under conditions of darkness. Melatonin is believed to act as The Earth rotates around its axis and as a result, all creatures the main internal synchronizer of the peripheral clocks with the exposed to daylight on Earth experience 24-h cycles of light and SCN and among themselves by transmitting time information dark. Biological rhythms are self-sustaining oscillations with a set from the master biological clock to the peripheral clocks. of species-speciﬁc characteristics, including amplitude, phase and Maintaining the sequential and phase-relation ordering of the period. Living organisms have adapted to this daily rotation of the various circadian rhythms from molecular to behavioral levels is

1Lighting Research Center, Rensselaer Polytechnic Institute, Troy, NY, USA and 2Pediatrix Medical Group, Memorial Hospital, South Bend, IN, USA. Correspondence: Dr MG Figueiro, Lighting Research Center, Rensselaer Polytechnic Institute, 21 Union Street, Troy, NY 12180, USA. E-mail: fi[email protected] Received 9 July 2012; accepted 8 January 2013 Shift work health consequences and design implications MG Figueiro and RD White S18 crucial for coordinated function throughout the human body. As normal melatonin hormone production at night. A second theory discussed below, chronic desynchrony between the master focuses on the negative health effects of light-induced circadian biological clock and peripheral clocks (or circadian disruption), disruption. which can be experienced by rotating shift workers, can impact health and well-being. MELATONIN, LAN AND CANCER The hormone melatonin is synthesized by the pineal gland LIGHTING CHARACTERISTICS AFFECTING CIRCADIAN at night and under conditions of darkness in mammals.14 The RHYTHMS synthesis and release of melatonin follow a robust circadian Light is formally defined as optical radiation reaching the human rhythm and are highly governed by the light/dark cycle. retina that provides visual stimulation. All current light sources Substantial laboratory evidence supports an important role for and light meters are calibrated based on the characteristics of LAN in cancer etiology through the melatonin pathway. Carillo- light that affects our visual system. There are five important Vico et al.15 have shown that melatonin participates in various characteristics of light for both the human visual and circadian physiological processes, including immune system functions. systems: quantity, spectrum, timing, duration and distribution. Since the early 1980s, evidence from experimental studies The neural machinery in the mammalian retina provides light suggests a link between melatonin and tumor suppression. information to both the visual and circadian systems, but the two Melatonin can prevent damage to DNA, and DNA that is not systems process light differently.3 Rods, cones and a newly repaired can mutate and initiate cancer.16,17 Reports show discovered photoreceptor, the intrinsically photosensitive retinal that melatonin is oncostatic in a variety of tumor cells: in vitro ganglion cells,4 participate in circadian phototransduction (how studies, although not entirely consistent,18 indicate that both the retina converts light signals into neural signals for the pharmacological and physiological doses of melatonin reduce biological clock). The quantity of polychromatic ‘white’ light the growth of malignant cells of the breast19–23 and other necessary to activate the circadian system is at least two orders of tumor sites.24–28 magnitude greater than the amount that activates the visual Blask et al.29 showed that exposure to different levels of white system. The circadian system is maximally sensitive to short- fluorescent light during the 12-h dark phase resulted in a dose- wavelength (‘blue’) light, with a peak spectral sensitivity at around dependent suppression of melatonin in rats bearing rat 460 nm,5–7 while the visual system is most sensitive to the middle- hepatomas or human breast cancer xenografts. Further, they wavelength portion of the visible spectrum at around 555 nm.3 showed that exposure to increasing levels of white light resulted Operation of the visual system does not depend significantly on in dose-dependent stimulation of tumor growth and linoleic acid the timing of light exposure, and thus responds well to a light uptake and metabolism. Relatively low light levels suppressed stimulus at any time of the day or night. On the other hand, the nocturnal melatonin in these rats. Blask et al.29 also showed that circadian system is dependent on the timing of light exposure.8,9 human nocturnal melatonin signals inhibit activities such as Light after the minimum core body temperature (CBTmin) will linoleic acid uptake that are associated with human breast cancer advance the timing of the biological clock (that is, promote earlier growth and that this effect is diminished by ocular exposure to bedtimes) and light before the CBTmin will delay the timing of the bright, white LAN. They exposed women to 2800 lux of white light biological clock (that is, promote later bedtimes). CBTmin typically at eye level, which resulted in an B40% reduction of melatonin occurs about 1.5 to 2 h before natural awakening. In addition, levels in blood compared with darkness. Perfusing tumors while the visual system responds to a light stimulus very quickly bearing human breast cancer xenografts with this melatonin- (in ms), the duration of light exposure needed to stimulate the depleted human blood resulted in increased linoleic acid uptake circadian system can take minutes.3 For the visual system, spatial in the xenografts, which is related to increased cancer growth.29 light distribution is critical for good visibility. It is not yet well Although this landmark study is limited to an indirect relationship established how light incident on different portions of the retina between melatonin and cancer growth rates, melatonin depletion affects the circadian system, although one study showed that light might also have important implications for cancer development.30 reaching the lower retina was more effective in suppressing melatonin than light reaching the upper retina.10 It is also important to note that the short-term history of light exposure CIRCADIAN DISRUPTION AND CANCER affects the sensitivity of the circadian system to light; the higher A person whose rhythms are synchronized to diurnal activity by a the exposure to light during the day, the lower the sensitivity of light/dark cycle and their social activities must undergo phase the circadian system to light at night (LAN), as measured by readjustment when forced to adapt to a new light/dark cycle (or nocturnal melatonin suppression11 and phase shifting.12 sleep/wake cycle, in the case of night-shift workers). The SCN and peripheral clocks will readapt to the new light/dark cycle, but it requires several days for a complete phase shift. Moreover, the SHIFT WORK, LAN AND CANCER RISK timing it takes for each of the different peripheral clocks to re- In 2007, the International Agency for Research on Cancer classified entrain to the new light/dark cycle varies, as different clock genes shift work that involved circadian disruption as ‘probably respond faster to the changes than others.31 As a result, an carcinogenic to humans’.13 This conclusion was based on data internal desynchronization occurs between the SCN, which is from animal studies, which show that light exposure during the faster to readjust to the new light/dark cycle, and the peripheral dark period that leads to acute melatonin suppression and/or clocks, as well as among the different peripheral clocks. This circadian disruption is associated with the development and internal desynchronization (or circadian disruption) affects the growth of tumors. Although the International Agency for Research entire body, including sleep efficiency, alertness and physical on Cancer decided that data from animal models was sufficient to and performance efficiency. More importantly, it is believed conclude that there was a causal link between shift work and that this internal desynchronization affects cell metabolism and circadian disruption resulting from exposure to LAN, they felt that proliferation.32 In fact, Filipski et al.33,34 showed that tumor growth the epidemiological evidence for the carcinogenicity of light was faster in mice subjected to repeated alteration of light/dark during the daily dark period was limited. cycles (chronic jet lag), resulting in a severe alteration of activity There are two parallel theories concerning the relationship and CBT rhythms. In humans, lower morning urinary melatonin between shift work, LAN exposure and hormone-related diseases levels, which are more likely to be experienced by night workers, such as breast cancer. One theory focuses on LAN as a disruptor to have been associated with an elevated breast cancer risk.35

Journal of Perinatology (2013), S17 – S23 & 2013 Nature America, Inc. Shift work health consequences and design implications MG Figueiro and RD White S19 Moreover, Fu and Lee36 demonstrated that the disruption of clock suggests a 50% increased risk of breast cancer associated with gene function might increase cancer risk because clock genes may night work.45 help maintain important cell repair mechanisms, such as the It has also been shown that patterns of night-shift work may apoptosis of damaged cells. You et al.37 showed that Per2 be a variable associated with breast cancer risk. Hansen and expression was arrhythmic in breast tumor cells, while a rhythm Stevens46 found a significant increased risk for breast cancer in of Per2 expression was found in healthy mammary tissues by nurses who work rotating shifts after midnight compared with Metz et al.38 those who work permanent day shifts. Those who worked evenings, however, were not at higher risk than day shift workers. Of all the rotating shift patterns investigated, Hansen SHIFT WORK AND CANCER RISK and Stevens46 found that the highest breast cancer risk was Observational studies suggest that shift work is associated with an observed for long-term day–night rotating shifts. These results are increase in breast and colorectal cancer risk, potentially linked consistent with those from Lie et al.47 who showed a significant to melatonin suppression by exposure to LAN. A Finnish increased risk of breast cancer in nurses who worked at least record linkage study first reported a significant excess of breast 5 years with 6 or more consecutive night shifts per month. These cancer cases among the 1577 female cabin attendants who were two studies suggest that breast cancer risk may be proportional to followed for 26 years.39 This increased risk was most prominent the number of consecutive night shifts worked. after 15 years on the job. A second record linkage study from A few studies failed to show a relationship between rotating Iceland, which used employment time of female flight attendants shifts, which may be associated with increased melatonin as a surrogate exposure, reported similar findings: an increased suppression or circadian disruption and breast cancer risks. Pesch breast cancer risk after 15 years on the job.40 A third record et al.48 investigated the association between night work and linkage study in Norway assessed breast cancer incidence in cancer risks in a German population-based case–control study and female radio and telegraph operators with potential exposure to found that having ever done shift work was not associated LAN.41 Among the 2619 predominantly postmenopausal women with an increased breast cancer risk compared with women who who were followed for 30 years, an increased risk for breast cancer have only worked day shifts. Similarly, Pronk et al.49 investigated was described even after adjustment for age, calendar year and the association between shift work and breast cancer risk in a year of first birth. The risk for breast cancer was highest in women population-based, prospective cohort study of women in aged 50 years or over who were in the highest cumulative shift Shanghai, China and found no link between breast cancer risk work exposure category (versus no shift work). and night-shift work. In 2001, three studies were published that adjusted their risk Animal studies suggest that the antiproliferative effect of calculations for reproductive history.42–44 The smallest study, a melatonin is not limited to breast cancer.50–53 The few previous case–control study with 813 breast cancer cases and 793 controls, epidemiological studies that have addressed the relationship assessed details on sleep patterns and habits, lighting character- between shift work and cancer other than breast cancer suggest istics of the subject’s bedroom in the 10 years before diagnosis, an elevated cancer mortality related to shift work.54,55 Tynes and lifetime occupational history.43 The main findings of the study et al.41 reported an increased risk of colon and rectum cancer in were a positive and statistically significant association between their cohort of female radio and telegraph workers. Rafnsson shift work that included night work (starting work after 1900 hours et al.40 did not report risks for colorectal cancer among the female and leaving work before 0900 hours) and breast cancer as well as Icelandic flight attendants, but described an elevated risk for a 13% increase in risk per additional year having worked at least tumors of the lymphatic system. Similar to breast cancer risks, one night shift per week. The authors also described a modest findings based on the data from the Nurses’ Health Study raise the increase in breast cancer risk among women who reported not possibility that a longer duration of rotating night-shift work may sleeping during the period of the night when nocturnal melatonin increase the risk of colorectal cancer. In a large, prospective study levels are typically the highest, indicating that women with the of shift work and colorectal cancer, the risk of colorectal cancer brightest bedrooms (that is, likely the highest exposure to LAN) was significantly elevated in women who had worked for more had an increased risk of breast cancer development. All results than 15 years on rotating night shifts, compared with those who were adjusted for parity, first-degree family history of breast never worked rotating night shifts. cancer, oral contraceptive use and recent (within the past 5 years) discontinued use of hormone replacement therapy. The second study, a large, population-based case–control study, investigated LAN, HORMONE LEVELS AND SHIFT WORK breast cancer risk among Danish women 30 to 54 years old who As acute melatonin suppression by LAN has been implicated as an worked predominantly at night.42 Women were considered to endocrine disruptor in shift workers, recent studies have work predominantly at night when they had been employed for at compared melatonin levels among this group and those working least 6 months in one or more of the trades in which at least 60% day shifts. Grundy et al.56 examined light exposures and melatonin of the females had nighttime schedules. The authors of the study levels in 123 rotating shift nurses. Despite being exposed to identified 7035 women with breast cancer and reconstructed their significantly higher light levels during the working nights, peak individual employment histories back to 1964 by record linkage melatonin levels and daily change in melatonin levels were similar with the files of the nationwide pension fund. After adjustment for across day shifts and night shifts in these rotating shift workers. reproductive history (age at birth of first and last child and Peplonska et al.57 examined 6-sulfatoxymelatonin (aMT6s), a number of children) as well as socioeconomic status, women who measure of total nighttime melatonin secretion, in 354 nurses worked at night for at least 6 months had a 50% greater risk for and midwives and found no significant differences in aMT6s breast cancer. The authors further noted a positive trend with concentrations between women working on rotating shifts and increasing duration of work at night (no P-value was reported). those working on day shifts. Of those working on rotating shifts, The third of these studies utilized data from the Nurses’ Health workers who reported having worked, on average, 8 or more night Study.44 The increased risk associated with extended periods shifts per month had significantly lower aMT6s concentrations (30 or more years) of rotating night work was 36%, after than those who worked fewer nights per month. In another study, controlling for known breast-cancer risk factors. The risk Langley et al.58 found no significant association between levels of increased with a greater number of years on shift work. Finally, melatonin and sex hormones (oestradiol, oestrome, progesterone a meta-analysis summarizing current observational studies of and prolactin) among rotating shift workers. These results were night workers including several studies of flight attendants, consistent with those from women in the Nurses’ Health Study

& 2013 Nature America, Inc. Journal of Perinatology (2013), S17 – S23 Shift work health consequences and design implications MG Figueiro and RD White S20 cohort, whose melatonin levels were not associated with sex better sleep quality during the day and possibly better health hormone levels.59 Dumont et al.60 measured ambulatory light are some of the benefits associated with a circadian adaptation to exposure and 24-h melatonin production (aMT6s) in 13 full-time night-shift work. Smith et al.73 proposed a compromise solution, rotating shift workers working both night and day/evening shift where light is applied at the first half of the shift to delay the dim periods. The authors found no difference in total 24-h aMT6s light melatonin onset (DLMO) in rotating shift workers. DLMO excretion between the two working periods. Night shifts were occurs when melatonin levels rise above a certain threshold, associated with a desynchronization between the melatonin and typically about 2 h before sleeping. Evening light exposures would the sleep–wake cycles. Light exposures were not correlated delay the onset of melatonin, and thus, delay their sleep times, with aMT6s levels excreted during the night of work, but were turning shift workers into extreme ‘night owls.’ Although this negatively correlated with total 24-h aMT6s excretion over the compromise solution was never tested in the field, it is entire night-shift period. The authors suggest that circadian hypothesized that this extreme ‘night owl’ behavior would help desynchrony may have induced the overall lower levels of shift workers cope better with staying awake at night and still be melatonin excretion.60 awake during the daytime hours when they were off their shift. Although more work still needs to be performed, these initial As discussed above, in most studies to date, the alerting effects field measurement results do not rule out LAN as a potential risk of light have been linked to its ability to suppress melatonin.74 factor for cancer in shift workers, but they cast some doubt on the In a recent study, however, Figueiro et al.75 demonstrated that role of melatonin as a mediator between LAN and serum estrogen exposures to both short-wavelength (blue) and long-wavelength levels. (red) lights in the middle of the night increased beta power (more cognition) and reduced alpha power (less sleepiness) relative to preceding dark conditions, although only blue light significantly OTHER HEALTH ISSUES ASSOCIATED WITH SHIFT WORK suppressed melatonin relative to darkness. Exposures to high (that Although causality has not been established, shift work has is, 40 lux at the cornea), but not low (that is, 10 lux at the cornea), also been associated with increased risk of heart disease,61 levels of red and of blue light significantly increased heart rate ischemic stroke,62 depression,63 type 2 diabetes,64 obesity,65 relative to darkness.75 Figueiro and Rea76 demonstrated that both gastrointestinal dysfunction66 and reproductive problems.67,68 470-nm (blue) and 630-nm (red) lights can significantly increase nighttime cortisol levels, heart rate and performance (Figure 1). These findings suggest that melatonin suppression is not needed IMPLICATIONS FOR PRACTICE to modulate nighttime alertness. These results are consistent with In addition to the health risks discussed above, coping with night- studies showing alerting effects of light during the daytime, when shift work is harder as a result of the natural tendency to be asleep melatonin levels are low.77,78 at night. Light can acutely impact alertness in rotating shift Therefore, the combination of red light with low ambient white workers or it can phase shift the timing of their biological clocks, light might be a good alternative to maintain melatonin levels so that they are more entrained to their working shifts. Acute high at night and still provide an alerting stimulus to those trying effects are much easier to achieve because their benefits are to stay awake while the body is telling them to sleep. perceived shortly after the light exposures. Phase-shifting effects, on the other hand, allow shift workers to cope with being awake at night by providing entrainment to the night shift. Light IMPLICATIONS FOR NICU DESIGN exposure control throughout the 24 h is, however, needed to As the time frame between planning the design for an NICU until maintain entrainment to the night shift, making it harder for it is replaced is often 20 to 30 years, it is important to consider workers to comply with the new light regimen. The reversed whether the growing evidence of adverse health effects of night- schedule would only work for permanent shift workers, rather shift work might have implications for the design process that is than rotating shift workers who work 2 to 4 nights per week. going on now. We propose that important considerations do exist Studies have shown that acute exposure to high levels of bright in at least two categories: lighting for staff and enhanced design LAN (levels typically42500 lux at the cornea) increases alertness, for safety. measured subjectively and objectively. Badia et al.69 showed that It will be apparent that the visual and circadian needs of staff exposure to 90 min of 5000 to 10 000 lux of white light during the are likely to be quite different from those of patients. Whereas nighttime increased brain activities in the alerting range. Cajochen most babies can and should remain in a dim light environment at et al.70 also showed that exposure to 3190 lux at the eye of white night, some staff members may have difficulty staying alert in that light increased brain activities, and that much lower levels setting. Unit designs that provide no space for a nurse to perform of short-wavelength (blue) light increased brain activities and his/her charting or collaborate with colleagues away from the reduced sleepiness at night.71 In a field study, it was shown that bedside with lighting conditions most suitable for his/her needs 15-min exposure to 2500 lux on the workplane (about 500 lux at may endanger both the health of the nurse and the safety of the cornea) delivered via metal halide table lamps (white light) the baby. Alternatively, personal light treatment devices can be during break times improved subjective ratings and increased provided to each worker so that they can have individualized light tympanic temperatures of nurses working in a Newborn Intensive treatments (for example, light goggles delivering red light) while Care Unit (NICU).72 still at bedside. Phase shifting the circadian system using light has been shown We do not currently know what is the safest and healthiest to be an effective way to increase adaptation to the night shift lighting environment for night-shift workers. It is unlikely that it because the worker is no longer producing melatonin at night; will be the same for all staff regardless of age, visual acuity, ‘night rather, as a result of a 12-h phase shift, workers produce melatonin owl’ characteristics and even genetic susceptibility to cancer. In during the day, when they are supposed to be asleep. This is this setting, NICU design should create adequate space for staff certainly the preferred lighting solution because it eliminates any members to gather away from the infant care space, where adverse effects of being awake while the circadian clock is ready lighting levels can be adjusted to meet the needs of staff to go to sleep. However, lifestyle changes are required to members without impinging on the infant care space. Both work accomplish and maintain the shift in the timing of the biological and lounge spaces should allow a worker to choose an area where clock because light exposure needs to be controlled throughout the light level and spectrum are most suitable to his/her needs. the 24-h cycle. Improvement in certain types of performance tasks, Alternatively, the introduction of napping may help because it greater subjective and objective alertness and wakefulness, can dissipate some of the sleep pressure that is usually high in

Journal of Perinatology (2013), S17 – S23 & 2013 Nature America, Inc. Shift work health consequences and design implications MG Figueiro and RD White S21

Figure 1. Left: The matching to sample (MTS) throughput score (Tput) is based on an accuracy/speed ratio calculated by the performance vigilance test software. MTS Tput was greater after the red and blue light conditions, but not after the dark condition, when compared to initial measurements in darkness. Right: Same analysis for reaction time (RT) Tput; a higher Tput score signiﬁes better performance. A color reproduction of this ﬁgure is available on the Journal of Perinatology online.

suppression of melatonin needs to be determined outside laboratory conditions. Moreover, it is important to measure circadian disruption in the field. Rea et al.81 and Miller et al.82 proposed a metric (phasor magnitude) to quantify circadian disruption using light/dark and activity/rest patterns collected using a novel personal light meter device.82 Phasor magnitude is the correlation between circadian light exposure and activity. A higher magnitude indicates the subject has a consistent, 24-h schedule with respect to activity and light. Lower magnitudes indicate a low correlation between daily cycles of light and activity irrespective of phase differences. As shown in Figure 2, phasor magnitudes decrease as number of shift-work nights increases. Therefore, future work should use tools such as the circadian light meter83,84 and phasor analyses to determine optimum shift-work schedules to minimize disruption. Until we have a better grasp of the light stimulus and how it Figure 2. Phasor magnitudes for day shift (0 nights worked) and rotating shift nurses plotted as a function of number of nights may impact melatonin levels or promote circadian disruption in worked. The ‘‘all-at-once’’ phasor analyses technique computes the the field, it will be very hard to establish the link between circular correlation function of the entire data set (e.g., seven days) LAN, melatonin suppression, circadian disruption and cancer risks in one operation. The relationship between two sets of time-series in shift workers. data (light/dark and activity/rest) can be quantified through phasor analysis, which incorporates a fast Fourier transform power and phase analysis of the circular correlation function computed from the two data sets. If the power in the longer, infradian, or shorter, CONFLICT OF INTEREST ultradian, periods in the circular correlation is large, the magnitude Rensselaer Polytechnic Institute received speaking fees from Acuity Lighting on of the 24-h phasor is reduced. Graph adapted with permission from behalf of MGF. RDW is employed by Pediatrix and owner of WhiteBriar Corporation. Sage and from Miller et al.82

ACKNOWLEDGEMENTS night-shift workers. A brief sleep period has been identified as a We like to acknowledge Mark Rea, Rebekah Mullaney and Nicholas Hanford for potential strategy to improve performance, reduce fatigue and their technical and editorial assistance. The Office of Naval Research (award no. increase vigilance for individuals working extended hours or N000141110572) funded some of the work presented in this manuscript. during night shifts. Smith-Coggins et al.79 studied 49 emergency This publication has been sponsored by Lifespan Healthcare LLC, Philips Healthcare department nurses and physicians and showed that those in a nap and Pediatrix Medical Group. The sponsoring entities for this publication had no involvement in the preparation or alteration of the manuscripts. group experienced less fatigue and fewer performance lapses on night shifts than subjects in a control group. For restorative napping to occur, it is important that managers and workers have a safe and comfortable resting place and that the sleep latency REFERENCES (3- to 10-min period after awakening) is considered when 1 Beers TM. Flexible schedules and shift work: replacing the ’9-to-5’ workday? Mthly planning on allowing staff to take naps.80 Labor Rev (Bureau of Labor Statistics), 2000; 123:6. 2 Moore RY. Circadian rhythms: basic neurobiology and clinical applications. Annu Rev Med 1997; 48: 253–266. FUTURE RESEARCH 3 Rea M, Figueiro M, Bullough J. Circadian photobiology: an emerging framework Future research targeting the link between LAN and breast cancer for lighting practice and research. Light Res Technol 2002; 34: 177–190. 4 Berson D, Dunn F, Takao M. Phototransduction by retinal ganglion cells that set risk should focus on ecological measurements of light/dark and the circadian clock. Science 2002; 295: 1070–1073. activity/rest patterns. It is important to measure the 24-h light 5 Brainard GC, Hanifin JP, Greeson JM, Byrne B, Glickman G, Gerner E et al. Action exposures that rotating shift workers are being exposed to in the spectrum for melatonin regulation in humans: evidence for a novel circadian field and how it may impact melatonin levels. Threshold for photoreceptor. J Neurosci 2001; 21: 6405–6412.

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