Men and women sleep at a similar circadian phase.
Sleep 2011 Jun; 34(Abstract Suppl):A62
Introduction: Some studies have reported that women sleep at a slightly earlier clock time than men. Other studies have reported that women also have circadian clocks set to a slightly earlier time, as seen in the earlier onset of the endogenous melatonin rhythm collected in dim light (DLMO). However reports on whether women sleep at a different circadian phase to men are mixed: some report that the DLMO is earlier relative to sleep in women versus men (thus women sleep at a later circadian time). while others report no sex difference. Here we reexamined a large data set to look for sex differences in circadian phase angle. Methods: Forty four healthy subjects (19-43 years) maintained a fixed 8 hour baseline sleep schedule (verified with wrist actigraphy) for at least 3 days, determined by their average self-reported bed and wake times from the previous week. This was immediately followed by a circadian phase assessment during which half hourly saliva samples were collected in dim light "10 lux) for determination of the DLMO. Each man was matched to a woman (free of hormonal contraception) according to age (+/- 5 years) and wake time (+/- 30 minutes) as per Cain et al. 2010. Results: Women slept on average 12 minutes later relative to their circadian clock than men (DLMO to bedtime interval 3.0 versus 2.8 hours respectively. p=0.55). These results are in the same direction as reported by others, but show no significant sex difference in circadian phase angle. Conclusion: These results suggest that men and women sleep at a similar circadian phase. We are currently adding an additional 20 men-women matched pairs to this data set and so will soon have the largest data set available (n=42 matched pairs) to comprehensively examine sex differences in circadian phase angle.
Sleep-deprivation; Sleep-disorders; Circadian-rhythms; Humans; Men; Women; Age-groups
Rush University Medical Center, Chicago, Illinois