A late wake time phase delays the human dim light melatonin rhythm

Abstract

Short sleep/dark durations, due to late bedtimes or early wake times or both, are common in modern society. We have previously shown that a series of days with a late bedtime phase delays the human dim light melatonin rhythm, as compared to a series of days with an early bedtime, despite a fixed wake time. Here we compared the effect of an early versus late wake time with a fixed bedtime on the human dim light melatonin rhythm. Fourteen healthy subjects experienced 2 weeks of short 6h nights with an early wake time fixed at their habitual weekday wake time and 2 weeks of long 9 h nights with a wake time that occurred 3h later than the early wake time, in counterbalanced order. We found that after 2 weeks with the late wake time, the dim light melatonin onset delayed by 2.4 h and the dim light melatonin offset delayed by 2.6 h (both p < 0.001), as compared to after 2 weeks with the early wake time. These results highlight the substantial influence that wake time, likely via the associated morning light exposure, has on the timing of the human circadian clock. Furthermore, the results suggest that when people truncate their sleep by waking early their circadian clocks phase advance and when people wake late their circadian clocks phase delay.

Fig. 1

Mean dim light salivary melatonin profiles following 2 weeks with an early wake time (light line) and 2 weeks with a late wake time (dark line) with the same bedtime in both conditions. Error bars represent S.E.s. The horizontal line indicates the average threshold. The mean melatonin profiles were constructed by referencing each individual subject’s data to the time of their DLMO following the days with the early wake time.

Fig. 2

Mean light levels from medallion photosensor during the 2 weeks of short nights with the early wake time (filled squares) and 2 weeks of long nights with the late wake time (open circles), averaged into 30 min bins. Data are averaged according to time from bedtime and are corrected for sunglasses use. Error bars represent S.E.s. Asterisks indicate times when the light intensity was significantly different between the conditions (data log transformed, repeated measures ANOVA followed by simple main effect analysis, p < 0.001).