Abstract
While much research has investigated the effects of exogenous melatonin on sleep, less is known about the relationship between the timing of the endogenous melatonin rhythm and the sleep–wake cycle. Significant inter-individual variability in the phase relationship between sleep and melatonin rhythms has been reported although the extent to which the variability reflects intrinsic and/or environmental differences is unknown. We examined the effects of different sleeping schedules on the time of dim light melatonin onset (DLMO) in 28 young, healthy adults. Participants chose to maintain either an early (22:30–06:30 h) or a late (00:30–08:30 h) sleep schedule for at least 3 weeks prior to an overnight laboratory visit. Saliva samples were collected under dim light (<2 lux) and controlled posture conditions to determine salivary DLMO. The 2-h difference between groups in the enforced sleep–wake schedule was associated with a concomitant 1.75-h delay in DLMO. The mean phase relationship between sleep onset and DLMO remained constant (~2 h). The variance in DLMO time, however, was greater in the late group (range 4.5 h) compared to the early group (range 2.4 h) perhaps due to greater effect of environmental influences in delayed sleep types or greater intrinsic instability in their circadian system. The findings contribute to our understanding of individual differences in the human circadian clock and have important implications for the diagnosis and treatment of circadian rhythm sleep disorders, in particular if a greater normative range for phase angle of entrainment occurs in individuals with later sleep–wake schedules.
Highlights
In a society where many aspects of contemporary life conflict with our biological adaptation to the 24-h cycle of light and darkness (Rajaratnam and Arendt, 2001), an understanding of the relationship between the sleep–wake cycle and the endogenous circadian system is of considerable importance to promote good health, safety, and productivity
The aims of the present study were (1) to test the hypothesis that individuals maintained on a late sleep–wake schedule show delayed dim light melatonin onset (DLMO) time compared to individuals on an early sleep–wake schedule; (2) to test the hypothesis that individuals on the late sleep schedule would sleep and wake at an earlier circadian phase than morning types; and (3) to examine the variability in circadian phase in relation to sleep–wake time
The present study examined the relationship between timing of sleep and endogenous circadian phase, as defined by the timing of the melatonin rhythm (DLMO)
Summary
In a society where many aspects of contemporary life conflict with our biological adaptation to the 24-h cycle of light and darkness (Rajaratnam and Arendt, 2001), an understanding of the relationship between the sleep–wake cycle and the endogenous circadian system is of considerable importance to promote good health, safety, and productivity. Sleep disorders that are characterized by misalignment of endogenous circadian rhythms with the desired or required time for sleep are referred to as circadian rhythm sleep disorders (CRSDs; American Academy of Sleep Medicine, 2005; Reid and Zee, 2009). These disorders are prevalent and are associated with reduced quality of life (Okawa and Uchiyama, 2007). Prolonged and consolidated sleep is only possible when sleep occurs during a particular range of circadian phases coinciding with the biological night in humans (Dijk and Czeisler, 1995).
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