Controlled Patterns of Daytime Light Exposure Improve Circadian Adjustment in Simulated Night Work

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1 Controlled Patterns of Daytime Light Exposure Improve Circadian Adjustment in Simulated Night Work Marie Dumont, *,,1 Hélène Blais, * Joanie Roy, * and Jean Paquet * * Chronobiology Laboratory, Sacré-Cœur Hospital of Montreal, Montreal, Québec Canada, and Department of Psychiatry, Université de Montréal, Montreal, Québec, Canada Abstract Circadian misalignment between the endogenous circadian signal and the imposed rest-activity cycle is one of the main sources of sleep and health troubles in night shift workers. Timed bright light exposure during night work can reduce circadian misalignment in night workers, but this approach is limited by difficulties in incorporating bright light treatment into most workplaces. Controlled light and dark exposure during the daytime also has a significant impact on circadian phase and could be easier to implement in real-life situations. The authors previously described distinctive light exposure patterns in night nurses with and without circadian adaptation. In the present study, the main features of these patterns were used to design daytime light exposure profiles. Profiles were then tested in a laboratory simulation of night work to evaluate their efficacy in reducing circadian misalignment in night workers. The simulation included 2 day shifts followed by 4 consecutive night shifts ( h). Healthy subjects (15 men and 23 women; years old) were divided into 3 groups to test 3 daytime light exposure profiles designed to produce respectively a phase delay (delay group, n = 12), a phase advance (advance group, n = 13), or an unchanged circadian phase (stable group, n = 13). In all 3 groups, light intensity was set at 50 lux during the nights of simulated night work. Salivary dim light melatonin onset (DLMO) showed a significant phase advance of 2.3 h (± 1.3 h) in the advance group and a significant phase delay of 4.1 h (± 1.3 h) in the delay group. The stable group showed a smaller but significant phase delay of 1.7 h (± 1.6 h). Urinary 6-sulfatoxymelatonin (amt6s) acrophases were highly correlated to salivary DLMOs. Urinary amt6s acrophases were used to track daily phase shifts. They showed that phase shifts occurred rapidly and differed between the 3 groups by the 3rd night of simulated night work. These results show that significant phase shifts can be achieved in night workers by controlling daytime light exposure, with no nighttime intervention. Key words night work, light exposure, phase shifts, circadian adjustment, melatonin, 6-sulfatoxymelatonin, circadian rhythms, human Circadian disruption is prevalent in night workers. The most obvious problem is that night workers need to work during the resting phase of the endogenous circadian cycle and try to sleep during the rising phase of circadian wake propensity (Czeisler and Dijk, 1995; Eastman et al., 1995). This mismatch 1. To whom all correspondence should be addressed: Marie Dumont, Chronobiology Laboratory, Sacré-Coeur Hospital, 5400 Gouin Blvd. West, Montreal, Québec, Canada H4J 1C5; marie.dumont@umontreal.ca. JOURNAL OF BIOLOGICAL RHYTHMS, Vol. 24 No. 5, October DOI: / SAGE Publications 427

2 428 JOURNAL OF BIOLOGICAL RHYTHMS / October 2009 between rest-activity and endogenous circadian cycles often results in short and nonrestorative sleep in the daytime and low vigilance during night work (Akerstedt, 1984; Ohayon et al., 2002). Circadian disruption may also be involved in increased risks of other health problems in night workers, such as cancer and metabolic disorders (Davis and Mirick, 2006; Morikawa et al., 2007). In most workers, circadian rhythms do not adjust to the night schedule (Benhaberou-Brun et al., 1999; Eastman et al., 1995; Knauth et al., 1981; Roden et al., 1993; Sack et al., 1992; Weibel 1997). The conflicting environmental light-dark cycle is one of the main reasons evoked to explain the lack of circadian adjustment in night workers (Czeisler and Dijk,1995; Eastman et al., 1995), and interventions using controlled light and darkness exposure to improve this adjustment have been tested (Boivin and James, 2005; Burgess et al., 2002). As most night workers choose to delay their sleep episode until shortly after their night shift, these interventions usually seek to delay endogenous circadian rhythms. Bright light exposure during the night can produce large circadian phase delays, and this approach has been used successfully in both simulations and field studies of night work (e.g., Boivin and James, 2002; Horowitz et al., 2001; Smith et al., 2009). However, this solution cannot be applied to all work environments, and many workers find bright light exposure during the night rather unpleasant (Budnick et al., 1995). Darkness during daytime sleep and the use of dark sunglasses in the morning have also been proven effective at facilitating phase delays, at least after 5 to 8 consecutive night shifts (Crowley et al., 2003; Eastman et al., 1994). In a previous study, we found that only a minority (8 of 30) of night nurses showed some circadian adaptation, reflected by a significant shift of melatonin production to the daytime, even after many consecutive nights of work (Benhaberou-Brun et al., 1999). This adaptation was associated with better daytime sleep quality, suggesting that even partial circadian adjustment could be beneficial to night workers. In the same study, light exposure was continuously recorded with ambulatory wrist monitors. We found very distinctive 24-h light exposure profiles between nurses who showed no circadian adaptation and those who showed phase-delay adaptation (n = 5) or phase-advance adaptation (n = 3) (Dumont et al., 2001). Interestingly, most differences in light exposure profiles occurred in the daytime. The timing of increased light exposure was globally consistent with predictions made by phaseresponse curves for light in humans (Khalsa et al., 2003; Minors et al., 1991), with increased light exposure (subjects not wearing sunglasses) in the morning in nurses who showed a phase advance and increased light exposure in the evening for those who showed a phase delay. In addition, both advanced and delayed nurses slept in darker bedrooms than nurses without circadian adaptation. These observations suggest that manipulating daytime light exposure profiles without changing light exposure during night work can produce a significant circadian adjustment in night workers. In the present study, we used the data from the previous study on nurses to design 2 experimental light profiles, and we tested their efficacy to produce circadian phase shifts in simulated night work. In addition to 1 profile aiming at a phase delay, we designed a 2nd light profile specifically for night workers who prefer to sleep before their night shift and therefore need to advance their circadian rhythms to achieve circadian adjustment. To avoid the inconveniences associated with bright light exposure, especially at night, we used a constant low light intensity during the night for both profiles, and we modulated light intensity and darkness exposure during the daytime only, at approximate levels normally measured in the field. The effects of these 2 experimental profiles on the circadian phase of salivary melatonin production were compared with the effects of a control profile representing a typical 24-h light profile of night nurses who showed no circadian adjustment. In addition, the circadian rhythm of urinary 6-sulfatoxymelatonin (amt6s) excretion was used to track daily circadian phase changes throughout the 4 nights of simulated night work and estimate the time course of expected phase shifts. Subjects MATERIALS AND METHODS Thirty-eight subjects, aged 20 to 35 years, participated in the study. They were divided into 3 groups matched for age, sex, and chronotype (MEQ score; Horne and Ostberg, 1976). One group was exposed to the delaying light profile (delay group; 5 men/7 women, age: 26.7 ± 4.6 years, MEQ score: 51.8 ± 6.9), a 2nd group was exposed to the advancing light profile (advance group; 5 men/8 women, age: 26.5 ± 4.2 years, MEQ score: 52.2 ± 5.6), and a 3rd group was exposed to the control light profile (stable group; 5 men/8 women, age: 26.6 ± 4.2 years, MEQ score: 53.5 ± 8.4). All subjects were in good physical and psychological health, as

3 Dumont et al. / DAYTIME LIGHT EXPOSURE AND CIRCADIAN ADJUSTMENT TO NIGHT WORK 429 determined by questionnaires. None had sleep complaints. All subjects had a regular sleep schedule with habitual 7- to 9-h sleep duration, as reported in 7-day screening sleep diaries. Volunteers who had experienced transmeridian travel or night work in the past month were excluded. All subjects were nonsmokers. They were asked to refrain from all medications (except hormonal contraceptives) and recreational drugs for at least 1 month before the study. Of the 23 women studied, 10 were using hormonal contraceptives (2 in the delay group and 4 in each of the advance and stable groups), and the other 13 entered the study at various phases of the menstrual cycle. A urinary toxicology screening confirmed that they were drug-free on the 1st day of the experiment. Each subject signed a consent form approved by the hospital s ethics committee and received financial compensation. Study Protocol The study began with 5 days at home, during which the subjects had to maintain an 8-h sleep episode (± 1 h) with fixed bedtimes and wake times (± 30 min). Naps were forbidden. Target sleep schedules were close to each subject s habitual sleep times and were assigned according to the subject s MEQ score: h for morning types (n = 6), h for evening types (n = 3), and h for neither-types (n = 29). Each group included 2 morning types and 1 evening type. Compliance was verified by sleep diaries and by 24-h ambulatory recordings of light and activity (Actiwatch-L; MiniMitter/Respironics, Bend, OR). Subjects were then admitted to the chronobiology laboratory for the following week. The laboratory part of the study is depicted in Figure 1 for the 3 groups. Subjects were admitted to the laboratory 5 h before their scheduled bedtime on day 1. They kept to their target 8-h sleep schedule on the 1st 2 nights (D2-D3). Light intensity was set at below 15 lux on day 2, 50 lux at horizontal eye level on day 3, and below 2 lux during the 2 night-sleep episodes. For all 3 groups, night work simulation was scheduled from midnight to 0800 h during the next 4 nights (D4 to D7). Light intensity was kept constant at 50 lux during the 1st 3 nights of night-work simulation and set at below 15 lux on the 4th night. Daytime sleep episodes were scheduled by group: from 0900 to 1700 h for the delay and stable groups and from 1400 to 2200 h for the advance group. Daytime light profiles for each group were applied during the 1st 3 days of simulated night work (D4 to D6). All subjects were kept in dim light (<15 lux) for the last 24 h in the laboratory and were allowed only 3 h of sleep in the middle of the last day (D7). Light intensity was set at below 15 lux on day 2 and during the last 24 h in the laboratory for the measure of the unmasked onset of salivary melatonin production. Circadian phase was assessed with salivary melatonin at the beginning and end of the laboratory protocol and with urinary amt6s excretion throughout the week in the laboratory. The study was conducted in the summer (May to September) in Montreal, Canada (45 31 N) over 3 consecutive years. Daytime Light Exposure Profiles Light exposure profiles were modeled according to the 24-h light exposures recorded in a previous field study of night nurses (Dumont et al., 2001), in which differing light exposures were found in nurses showing advanced, delayed, or stable circadian phases after an average of 4.7 consecutive nights of work (see Fig. 2A). The modeled light profiles are presented in Figure 2B. Subjects in the advance group were exposed to bright light (1800 lux) from 0800 to 0900 h to mimic natural outdoor light during commuting. This was followed by moderate indoor light exposure (300/150 lux) from 0900 to 1400 h followed by an 8-h sleep episode in darkness (2 lux) scheduled from 1400 to 2200 h. Subjects were then exposed to dim indoor light (20 lux) between their sleep episode and night work simulation ( h). Subjects in the delay group were exposed to moderate outdoor light intensity (400 lux) from 0800 to 0900 h to mimic outdoor light while wearing sunglasses during commuting. Their 8-h sleep episode in darkness (2 lux) was scheduled from 0900 to 1700 h, followed by moderate indoor light exposure (150/300 lux) from 1700 to 2300 h. Subjects were then kept in dim indoor light (20 lux) during the hour prior to night work simulation ( h). Subjects in the stable group were exposed to bright light (1800 lux) from 0800 to 0900 h and slept in dim light (20 lux) from 0900 to 1700 h. This was followed by moderate indoor light exposure (150 lux) from 1700 to 2000 h and dim indoor light (20 lux) in the 4 h preceding night work simulation ( h). Light intensities 50 lux were administered to the subjects with a programmable light ceiling composed of 196 fluorescent lamps (F32 T8 841 model; peak wavelength: 545 nm) controlled by Lutron Graphik Eye Liaison 2.0 (Lutron Electronics Co. Inc., Coopersburg, PA). Each 1-inch-diameter rapid-start lamp produces 32 W of cool lamp color at 4100 K and is connected to a Lutron Hi-Lume 1% fluorescent dimming ballast. Neutral density filters with an 80% attenuation factor were installed below the lamps to obtain the required

4 430 JOURNAL OF BIOLOGICAL RHYTHMS / October 2009 light intensities. Light intensities 20 lux could not be achieved with the light ceiling and were obtained with incandescent lamps (peak wavelength: 750 nm) mounted on the walls. Illuminance (in lux) was recorded every minute by an ambulatory wrist monitor (Actiwatch-L) and, except during the sleep episodes, was measured every hour in the angle of gaze by a photometer (Lutron, LX-1108; Duncan Instruments Canada Ltd., North York, ON, Canada). Light exposure measured in lux is not adjusted to the specific spectral sensitivity of the circadian system (Brainard et al., 2001; Thapan et al., 2001), but it allowed us to match the light intensity profiles measured in the night nurses of our previous study (Dumont et al., 2001). For each subject, light data from the ambulatory monitors were log-transformed and averaged over each hour of recording. Circadian Phase Assessments Two markers were used to estimate circadian phase. The main marker was the onset time of melatonin production (dim light melatonin onset, DLMO) measured in the saliva on day 2 and during the last 24 h (D6 and D7) in the laboratory. The 2nd marker was the acrophase of urinary amt6s concentration determined from day 2 to day 7. Salivary Melatonin Figure 1. Illustration of the laboratory study protocol for the 3 groups of subjects. Light intensity (in lux) is indicated for each clock hour of the 7 days (D1 to D7) spent in the laboratory. Scheduled sleep times are illustrated with moon crescents. Sleep times from D1 to D3 are illustrated for neither-type subjects. Sleep and light schedules from D4 to D7 were the same for all subjects in each group, irrespective of chronotype. Phase assessments using salivary melatonin were conducted in dim light (<15 lux) on day 2 and during the last 24 h in the laboratory (D6 and D7). Saliva samples were collected every 30 min during the 6.5 h preceding bedtime on day 2 (13 samples). Saliva samples were also collected every half-hour during the last 24 h in the laboratory (starting at 2200 h on day 6), except during the 3 h of sleep allowed in the middle of the afternoon (49 samples). Saliva was collected in dim light (<15 lux) using Salivettes (Sarstedt Inc., Montreal, Qc, Canada). During saliva collection periods, subjects were seated and allowed to do quiet activities such as

5 Dumont et al. / DAYTIME LIGHT EXPOSURE AND CIRCADIAN ADJUSTMENT TO NIGHT WORK 431 Figure 2. (Top panels, A) Light exposure measured with ambulatory wrist monitors in night nurses who showed an advanced (n = 3), stable (n = 22) or delayed circadian phase (n = 5). Recordings from 2400 to 0800 h were made during night work. Data (mean log lux and sem) were averaged over 2 consecutive 24-h periods on every hour (redrawn from Dumont et al., 2001). (Middle panels, B) Experimental daytime light profiles designed for the 3 groups of subjects during the laboratory simulation of night work (day 4 to day 6). Light exposure was set at 50 lux for all 3 groups during night work ( h). Gray areas represent the time of scheduled sleep episodes. (Lower panels, C) Light exposure (mean log lux and SEM) measured with ambulatory wrist monitors during the 3 days of laboratory simulation of night work (day 4 to day 6) for the 3 groups of subjects. reading or watching TV. Each sample was immediately centrifuged and frozen at 20 C. Melatonin concentration in saliva was determined in duplicate using the Bühlmann Direct Saliva Melatonin ELISA (ALPCO Diagnostics, Windham, NH). Coefficients of variation (CVs) were 6.5% at 3.1 pg/ml for intra-assay and 7.5% at 3.0 pg/ml for interassay. Salivary DLMO was found by interpolation, with the onset being defined as the time at which melatonin concentration would exceed 3.0 pg/ml (Benloucif et al., 2008), except for 2 subjects with very high melatonin concentrations (onset defined at 10.0 pg/ml) and 1 subject with very low concentrations (onset defined at 1.3 pg/ml). Salivary DLMO could not be determined for 3 subjects in the advance group, 2 subjects in the stable group, and 2 subjects in the delay group, because of an amplitude too low (4 subjects), large variations with no discernible onset (1 subject), or technical problems during the assay (1 subject). Urinary 6-Sulfatoxymelatonin (amt6s) Subjects were asked to empty their bladder just before bedtime on day 1. Urine was then collected every 2 h from wake time on day 2 until 2200 h on day 7, except during sleep episodes, when the interval was 8 h. For each collection, total volume was measured and 5-mL aliquots were frozen at 20 C. When urine was

6 432 JOURNAL OF BIOLOGICAL RHYTHMS / October 2009 Table 1. Circadian phase at the beginning and end of night work simulation, and average circadian phase shifts assessed with salivary (DLMO) and urinary (amt6s) estimates of melatonin production (mean ± SD). Advance Group Stable Group Delay Group DLMO day 2 (h:min) 21:15 ± 1:19 22:16 ± 1:10 21:53 ± 1:31 DLMO day 7 (h:min) 18:59 ± 1:03 23:58 ± 2:26 01:58 ± 1.35 DLMO phase shift (h) 2.26 ± ± ± 1.33 amt6s acrophase day 2-3 (h:min) 06:23 ± 1:24 06:20 ± 1:15 06:22 ± 1:25 amt6s acrophase day 6-7 (h:min) 03:45 ± 1:49 07:08 ± 2:11 09:23 ± 2:03 amt6s phase shift (h) 2.64 ± ± ± 2.35 NOTE: For DLMO estimates, the number of subjects was 10 in the advance group, 11 in the stable group, and 11 in the delay group. For amt6s acrophase estimates, the number of subjects was 13 in the advance group, 11 in the stable group, and 10 in the delay group. DLMO = salivary dim light melatonin onset; amt6s = urinary 6-sulfatoxymelatonin. collected during awakenings within a sleep episode, it was added to urine collected at wake time and included in the sleep episode sample. Urinary amt6s concentration was determined in duplicate using Bühlmann 6-Sulfatoxymelatonin ELISA (ALPCO Diagnostics). Intra-assay CVs for control samples were 2.4% for 3.1 ng/ml and 7.0% for 22.2 ng/ml. Interassay CVs at 4.1 and 19.5 ng/ml were 8.7% and 8.3%, respectively. The amt6s acrophase was calculated by cosinor analysis (Nelson et al., 1979; Monk and Fort, 1983) using 48-h moving windows (Gibbs et al., 2007). Statistically nonsignificant cosinor fits (11/190, 5.5%) were rejected and led to the exclusion of 4 subjects (2 subjects each in the stable and delay groups). Data Analysis Circadian phase shifts were defined as the difference between salivary DLMO on day 7 and day 2, and between amt6s acrophase on day 6-7 and day 2-3. Twoway ANOVAs with group as an independent factor and day as a repeated measure were performed on salivary DLMOs and amt6s acrophases. Simple effect analyses were performed when significant interactions were found and Tukey s post hoc HSD tests were used to compare multiple means for significant main effects. When repeated measures with more than 2 levels were used, the Huynh-Feldt correction for sphericity was applied, but only the original degrees of freedom are reported. Pearson correlations between phase shift and initial phase and between phase shift and MEQ score were computed within each group. Intraclass correlation was also calculated between salivary and urinary phase shift estimates. Light exposures measured with the wrist monitor were averaged for each hour over the 3 days of light treatment (days 4-6), then summed for the 24 h. This total daily light exposure was compared among the 3 groups with a 1-way ANOVA. Statistical significance was set at Light Exposure Profiles RESULTS Averaged light exposure profiles measured with the wrist monitor in the laboratory are illustrated in Figure 2C. Total light exposure during the 3 days of light treatment (days 4-6) was similar for the 3 groups of subjects: 25.2 ± 3.5 log lux in the advance group, 24.3 ± 4.5 log lux in the stable group, and 25.1 ± 3.4 log lux in the delay group (F 2,35 = 0.22, p = 0.80). Salivary Melatonin Phase estimates (DLMO) on day 2 and day 7 and average phase shifts for each group are presented in Table 1. The 2-way ANOVA performed on DLMO revealed a significant group day interaction (F 2,29 = 52.8, p < 0.001), with both the delay and stable groups showing significant phase delays (p < for each group) and a phase advance (p < 0.001) for the advance group. Simple effects analyses showed that the phase delay obtained in the delay group was significantly larger than the phase delay observed in the stable group (F 1,29 = 15.3, p < 0.001). The 3 groups were not significantly different on day 2 (F 2,29 = 1.52, p = 0.24), but differed from each other on day 7 (F 2,29 = 41.3, p < 0.001). Individual DLMOs estimated at baseline (day 2) and on the 4th day of simulated night work (day 7) are presented in Figure 3. Urinary 6-Sulfatoxymelatonin (amt6s) Average acrophases estimated on day 2-3 and day 6-7 and average phase shifts for each group are presented in Table 1. The 2-way ANOVA showed a significant group day interaction (F 2,31 = 25.6, p < 0.001), with a significant phase delay in the delay group (p < 0.001) and a significant phase advance in the

7 Dumont et al. / DAYTIME LIGHT EXPOSURE AND CIRCADIAN ADJUSTMENT TO NIGHT WORK 433 advance group (p < 0.001). The stable group showed a nonsignificant phase delay (p = 0.18). The delay obtained in the delay group was significantly larger than the delay observed in the stable group (F 1,31 = 7.0, p = 0.01). The amt6s acrophases of the 3 groups were similar on day 2-3 (F 2,31 = 0.005, p = 0.99) but differed from each other on days 6-7 (F 2,31 = 23.2, p < 0.001). As shown in Table 1, the phase shifts estimated with salivary DLMO and urinary amt6s acrophases were not identical, but the differences were not statistically significant (p > 0.08 in all 3 groups). The intraclass correlation (ICC) computed between both phase-shift estimates was highly significant (ICC = 0.865, p < 0.001, n = 29). Amplitude and mesor estimated with the cosinor analyses showed no significant effects for group, day, or group day interaction (p > 0.23 for all). The daily progression of mean circadian phase for the 3 groups of subjects throughout the laboratory study is illustrated in Figure 4. Complete data were available for 9 subjects in the delay group and for 11 subjects in each of the advance and stable groups. Five subjects were missing only 1 cosinor fit, 1 in the delay group (days 3-4), 2 in the advance group (both on days 5-6), and 2 in the stable group (both on days 6-7). For those subjects, missing values were replaced by Yates method (Kirk, 1968). Therefore, analyses on daily progression included all subjects, except for 2 subjects in the delay group. There was a significant group day interaction (F 8,132 = 22.14; p < 0.001), and simple effects analyses showed that the amt6s acrophases started to differ on day 4-5 (F 2,33 = 11.20, p < 0.001) between the delay and advance groups (p< 0.05). Group differences were larger on day 5-6 and day 6-7 (F 2,33 = and 24.56, respectively; p < 0.001), with each of the 3 groups differing significantly (p < 0.05 for all comparisons). Correlations with Phase Shifts The only significant correlation between initial DLMO and phase shifts was found in the advance group (r = 0.67, p < 0.05, n = 10), showing that subjects with a later initial phase had a larger phase advance. There was no significant correlation between phase shifts and initial amt6s acrophase (p > 0.14) or with the MEQ score (p > 0.23) in any of the 3 groups. DISCUSSION Our results show that significant circadian phase shifts can be achieved in a simulation of night work by controlling light exposure in the daytime and without any intervention during the night. The magnitudes of Figure 3. Individual estimates of the timing of salivary dim light melatonin onset (DLMO) measured at baseline (D2) and after 3 night shifts, on the 4th day of simulated night work (D7). Group means (± SEM) are illustrated by filled symbols. the phase shifts obtained with our daytime light profiles compare with the results of interventions using bright light exposure during night work to achieve partial circadian adjustment to night work (Lee et al., 2006, Smith and Eastman, 2008). Partial adjustments facilitate the return to daytime schedule after a series on night shifts, and can reduce circadian disruption associated with repeated phase shifts (Haus and Smolensky, 2006). Partial circadian adjustments with similar phase delays (Crowley et al., 2004) or phase advances (Santhi et al., 2008) have been shown to improve vigilance during night work, and may also improve daytime sleep quality (Benhaberou-Brun et al., 1999).

8 434 JOURNAL OF BIOLOGICAL RHYTHMS / October 2009 Figure 4. Daily progression of the acrophase of urinary amt6s throughout the study for the 3 groups of subjects (mean and SEM), estimated by cosinor analysis on 48-h moving windows. Delay and advance groups differed on day 4-5 and the 3 groups differed from each other on day 5-6 and day 6-7 (p < 0.05 for each comparison). Analyses included 13 subjects each for the advance and stable groups and 10 subjects for the delay group. amt6s = 6-sulfatoxymelatonin. Not having to use bright light exposure during the night shift is an important advantage, because many work environments are not suitable for this kind of intervention, and many workers are reluctant to increase light intensity during the night (Budnick et al., 1995). Light exposure during the night is also thought to have negative health effects (Davis et al., 2001). On the other hand, significant phase shifts can be produced with ordinary room light during the night when subjects are exposed to very dim light during the day (<15 lux) (Boivin and Czeisler, 1998). However, constant low light exposure may increase mood disorders in night workers (McLaughlin et al., 2008). Our experimental light profiles did not change total light exposure compared with light levels typical of night workers showing no circadian adjustment. Our profiles only modulated the timing of various light and dark exposures. On average, subjects in the stable group showed a significant phase delay of their circadian phase. A similar delay in the absence of intervention was reported in previous studies (Boivin and James, 2002; Smith et al., 2009), reflecting the behavior of only a subset of subjects. In the present study, only half of the subjects in the stable group had a later phase in posttreatment than in baseline (Fig. 3). By contrast, in the delay group, all subjects showed a phase delay after light treatment (Fig. 3), and the average phase delay was more than 2 times larger than in the stable group. This larger effect was obtained even though both groups had exactly the same daytime sleep schedule. The only difference between the 2 groups was the daytime light exposure pattern, including light intensity during daytime sleep (Fig. 1). Few studies have examined the possibility of advancing instead of delaying circadian rhythms to improve adaptation to night work (Mitchell et al., 1997; Santhi et al., 2005, 2008). Most night workers choose to have their main sleep episode shortly after the end of their night shift, thereby delaying their sleep schedule by 8 to 10 h. Generally, therefore, it is more appropriate to use scheduling strategies that foster circadian adaptation by phase delay. However, as observed in 3 of the 30 nurses in our previous study (Dumont et al., 2001), some individuals prefer to advance their sleep schedule. This strategy is often preferred by morning-type individuals (Dumont et al., 2001; Natale et al., 2003) and by older night workers for whom the morning hours are a highly valued leisure time (Knauth and Costa, 1996). After 3 night shifts, our experimental advancing light profile produced a significant phase advance (2.3 ± 1.3 h) that was larger than that obtained with sleep-dark scheduling alone (0.9 ± 0.5 h; Santhi et al., 2005) and similar to the effects of combined sleepdark scheduling and bright light treatment (2.3 ± 0.6 h; Santhi et al., 2008). Some features of our experimental daytime light profiles have already been shown to improve the circadian adaptation of night workers. The importance of darkness during daytime sleep for circadian adjustment has been recognized for some time (Eastman et al., 1994). The timing of the sleep-dark episode has been tested experimentally and its influence was found to be as important as the timing of bright light to improve circadian adaptation in night workers (Mitchell et al., 1997). This was confirmed in subsequent studies, where only minimal phase shifts could be achieved with bright light during night work without adding a fixed daytime sleep-dark schedule (Horowitz et al.,

9 Dumont et al. / DAYTIME LIGHT EXPOSURE AND CIRCADIAN ADJUSTMENT TO NIGHT WORK ). In one of the few field studies testing bright light exposure to improve circadian adaptation in real night workers, it was shown that up to 45% of circadian adjustment to night-shift work could be obtained simply by maintaining a regular sleep-dark schedule (Boivin and James, 2002). Two studies tested conditions using only regular sleep-dark scheduling as an intervention (Horowitz et al., 2001; Santhi et al., 2005). After 3 consecutive nights of simulated night shifts, both studies found that a regular sleep-dark schedule produced circadian phase shifts, although these phase shifts were relatively small (about 2 h), similar to the average phase delay observed in our stable group. Attenuation of morning light exposure has also been proven effective in facilitating phase delays. This was observed in field studies that measured light exposure patterns in relation to circadian phase in night workers (Koller et al., 1994; Dumont et al., 2001) and was demonstrated in night work simulations (e.g., Crowley et al., 2003; Eastman et al., 1994). One simulation study included a condition similar to that for our delay group: subjects wore normal sunglasses during the travel-home window and had their sleep-dark episodes scheduled at 0830 h. Subjects showed an average phase delay of 2.5 h (± 2.5) after 5 night shifts (Crowley et al., 2003). We obtained a larger phase delay with less variability (4.1 h ± 1.3) after only 3 night shifts with our delaying light profile. This finding supports our proposal that using a complete daytime light profile (including higher evening light exposure for the delay profile) can significantly increase both the magnitude and the rate of circadian adjustment to night work. The light profile of our delay group was designed to mimic the light attenuation typical of normal sunglasses (~20%) used by real night workers commuting by car (Dumont et al., 2001). Although very dark sunglasses can increase the magnitude of phase delays, they are not safe for driving (Crowley et al., 2003). It was recently discovered that circadian photoperception is especially sensitive to shorter wavelengths (Brainard et al., 2001; Thapan et al., 2001). Blue blocker glasses have recently been designed to take advantage of this specificity and are being tested in studies on circadian adaptation to night work. The orange (Sasseville et al., 2006) or brown-looking (Lee et al., 2006) lenses of these glasses strongly attenuate the blue portion of the light spectrum, which amounts to placing the circadian system almost in the dark while allowing enough light transmission for safe driving. We would recommend using such glasses to increase the efficacy of a delaying daytime light profile. Urinary amt6s acrophases were well correlated with salivary DLMOs, a finding that confirms the good validity of this measure for night work (Ross et al., 1995). It is noteworthy that this good correlation was obtained even though all salivary samples were collected in controlled dim light (<15 lux), whereas urinary samples were subjected to the masking effects of various light intensities. Urinary data were extremely useful to track the daily progress of circadian phases (Fig. 4). They revealed that circadian shifts occurred rapidly after the 1st night shift and were significant by the 2nd night of work (day 4-5). The circadian phase then appeared to stabilize, reaching a plateau during the 3rd night (day 5-6). Longer studies with more consecutive night shifts would be required to verify whether a stable phase would be achieved or whether further adaptation would occur. In conclusion, our results show that it is possible to produce significant circadian phase shifts (both advances and delays) in night work simulations using only daytime manipulations of light-dark exposure. It can be expected that larger and/or faster phase shifts would be achieved by adding timed bright light exposure during the night. However, our results show that bright light exposure during night work is unnecessary, at least for partial circadian adjustment. Our daytime light profiles should be tested in the field where light exposure cannot be as tightly controlled as in a laboratory simulation and where light levels could be higher during night shifts, especially in industrial settings. However, because the light exposure profiles were designed according to actual light patterns measured in real workers, we believe that they could be successfully used by many night workers. ACKNOWLEDGMENTS This study was supported by research grants from the Canadian Institute of Health Research (MD) and the NSERC Undergraduate Student Research Awards Program (JR). The authors are grateful to Brahim Selmaoui and Stéphanye Naud for the biochemical assays, and to Geneviève Picard for the preliminary data analyses. REFERENCES Akerstedt T (1984) Work schedules and sleep. Experientia 40:

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