Early versus late bedtimes phase shift the human dim light melatonin rhythm despite a fixed morning lights on time

Short sleep/dark durations are common in modern society. In a within subjects counterbalanced design, we compared the effect of 7-19 nights with a late bedtime (01:00) versus 7-19 nights with an early bedtime (22:00) on the human dim light melatonin rhythm. All ten healthy subjects experienced lights on time at 07:00 and at least 5 min of outdoor light between 07:00 and 08:00. We found that despite the morning light exposure, the melatonin rhythm following the late bedtime nights was delayed by approximately 0.6 h (P<0.05), as compared to after the early bedtime nights. These results suggest that when people truncate their sleep by delaying their bedtime they also significantly phase delay their circadian rhythms and/or when people advance their bedtime their circadian rhythms advance.