Twenty-six participants completed the study, 13 in the circadian alignment group and 13 in the circadian misalignment group. The flow chart of enrollment is shown in Supplementary Figure 1, and Table 1 summarizes demographic data.
At baseline, total sleep time and amounts of SWS and REM sleep were similar in the two groups ( Table 1 , see Figure 2 for daily total sleep time). Our experimental strategy (Figure 1) consisted of increasing sleep pressure using a first aligned night of bedtime restriction to achieve similar levels of total sleep time during the period of bedtime restriction, irrespective of the presence or absence of circadian disruption. This strategy was successful, because the difference in total sleep time achieved during the seven periods of short sleep undisturbed by blood sampling averaged 22 min (i.e., ~3 min per bedtime period; Table 1 and Figure 2).
Total sleep times achieved on each day for both study groups. Represented values are mean (SEM).
Consistent with previous studies,[27,28] SWS was better preserved than REM sleep when bedtimes were restricted. Amounts of SWS and REM sleep were similar in both groups at baseline and during the 1-week intervention Table 1 .
Participants in the circadian alignment protocol (Figure 3) experienced a nonsignificant delay of the DLMO of 30 min (0, 60) whereas those exposed to circadian misalignment had a delay of 3 h 08 min (2 h 00 min, 3 h 30 min; P = 0.001). Of note, two men exposed to circadian misalignment did not shift circadian phase. The remaining 11 subjects shifted by 3 h 30 min (2 h 23 min, 3 h 38 min).
Assessments of circadian phase. Timing of DLMO before the first (•) and before the last (○) short sleep periods. The circadian phase could not be determined for one subject in the circadian misalignment group.
Weight Change and Caloric Intake
Participants were free to help themselves ad libitum during meals and had unlimited access to various snack items. In the circadian alignment group, breakfast was served between 0700 and 0830 h, lunch between 1300 and 1330 h, and dinner between 1830 and 1930 h. In the circadian misalignment group, on the days when bedtimes were scheduled during the day (R5–R6, R8–R9), a light meal (usually a sandwich) was served at 0100 h. A normal breakfast was served in the morning. Lunch was served at 1500 h and dinner was served between 1830 and 1930 h. Thus, there were only minimal differences in the timings of breakfast, lunch, and dinner between the two groups.
The two groups consumed excessive but almost identical amounts of calories, averaging 4,061 (971) Kcal/24 h when bedtime periods were aligned and 4,058 (888) Kcal/24 h when bedtime periods were misaligned. On average, daily caloric intake during sleep restriction included 62.8% (11.3%) carbohydrates, 33.0% (3.9%) fat, and 10.4% (2.1%) protein in participants in the aligned condition compared with 57.2% (5.2%) carbohydrates, 34.6% (3.8%) fat, and 10.8% (1.8%) protein in participants in the misaligned condition. None of the differences between the two conditions were significant (P < 0.115). On average, during the 7 days of sleep restriction, the proportion of calories consumed after 1900 h was 7% (4%) in the aligned condition versus 21% (8%) in the misaligned condition (P < 0.001). Weight gain was significant (P ≤ 0.002) and similar in both groups Table 1 .
SI decreased in 24 of the 25 participants after 7 days of sleep restriction. The findings were qualitatively similar in both groups, in that a robust decrease in SI was not compensated for by a commensurate increase in β-cell responsiveness (as assessed by AIRg), and therefore, the DI was decreased, consistent with an increased risk of diabetes Table 2 . Median hsCRP levels were higher after sleep restriction than at baseline in both groups, but the difference was significant for the misaligned group only Table 2 . These findings were similar when analyses were adjusted for weight change.
To examine the effect of the two interventions, we compared the percentage change in cardiometabolic variables between the two groups (individual data in Figure 4). After controlling for BMI, the interaction sex-by-intervention for percentage change in SI was −34% (23%) in the aligned group (n = 12) versus −47% (20%) in the misaligned group (n = 13), which was significantly different (P = 0.026). This interaction was not significant for percentage change in insulin secretion (+28% [55%] vs. +18% [36%], P = 0.16), DI (−17% [39%] vs. −39% [27%], P = 0.66), or hsCRP (+50% [67%] vs. +119% [110%]; P = 0.63). Because of the small number of women, the remainder of the analysis included only men (individual data in Figure 4). Figure 5 illustrates the glucose and insulin temporal profiles during the IVGTT under rested condition and after sleep restriction for the aligned and misaligned groups, and Figure 6 reports the changes in the summary measures derived from the IVGTT. Importantly, total sleep time for men only was not significantly different between the two groups during the sleep restriction period when baseline levels were controlled for (P = 0.30). The relative decrease in SI in men was nearly twice as large in the misaligned than in the aligned group (−58% [13%] vs. −32% [25%], P = 0.011; Figure 6). There were no compensatory increases in AIRg in either intervention group (+24% [39%] vs. +21% [56%], P = 0.66; Figs. 5 and 6). Therefore, the reduction in DI, reflecting an increase in diabetes risk, tended to be greater after circadian misalignment than when the sleep–wake cycle remained aligned (−48% [24%] vs. −19% [43%], P = 124; Figure 6). Increases in hsCRP after sleep restriction were higher in the misaligned than in the aligned groups (+146% [103%] vs. +64% [63%], P = 0.049; Figure 6) in male participants. Interestingly, these hsCRP increases were correlated with the shift in circadian phase, as estimated by melatonin onset (r = 0.487, P = 0.040).
Individual changes in cardiometabolic variables. Values represent percentage change from baseline of SI, AIRg, DI, and hsCRP.
Temporal profiles of glucose (top) and insulin (bottom) levels during IVGTT. Mean (SD) glucose and insulin levels during IVGTT performed under baseline (i.e., rested) condition and after 7 days of sleep restriction to 5 h per day for the men in the circadian alignment (n = 10) and the circadian misalignment (n = 9) groups. Visual examination of these profiles suggests that the effect of sleep restriction on the decline of glucose concentrations is greater in the presence of circadian misalignment despite higher levels of insulin, consistent with a greater decrease in SI. Minimal model analysis of individual profiles confirmed this visual impression.
Changes in cardiometabolic variables in male participants. Mean (SD) changes in SI, AIRg, DI, and hsCRP from baseline to sleep restriction are shown in both intervention groups. *P < 0.05.
In women, differences in IVGTT and hsCRP variables between the aligned and misaligned intervention were nonsignificant.
Diabetes. 2014;63(6):1860-1869. © 2014 American Diabetes Association, Inc.