Abstract
Limited research has compared the circadian phase-shifting effects of bright light and exercise and additive effects of these stimuli. The aim of this study was to compare the phase-delaying effects of late night bright light, late night exercise, and late evening bright light followed by early morning exercise. In a within-subjects, counterbalanced design, 6 young adults completed each of three 2.5-day protocols. Participants followed a 3-h ultra-short sleep-wake cycle, involving wakefulness in dim light for 2h, followed by attempted sleep in darkness for 1 h, repeated throughout each protocol. On night 2 of each protocol, participants received either (1) bright light alone (5,000 lux) from 2210–2340 h, (2) treadmill exercise alone from 2210–2340 h, or (3) bright light (2210–2340 h) followed by exercise from 0410–0540 h. Urine was collected every 90 min. Shifts in the 6-sulphatoxymelatonin (aMT6s) cosine acrophase from baseline to post-treatment were compared between treatments. Analyses revealed a significant additive phase-delaying effect of bright light + exercise (80.8 ± 11.6 [SD] min) compared with exercise alone (47.3 ± 21.6 min), and a similar phase delay following bright light alone (56.6 ± 15.2 min) and exercise alone administered for the same duration and at the same time of night. Thus, the data suggest that late night bright light followed by early morning exercise can have an additive circadian phase-shifting effect.
Highlights
Under everyday conditions, the circadian system is synchronized to the earth’s 24-h rotation to promote adaptation to the environment
The data suggest that late night bright light elicited a marginally greater circadian phase shifting effect than exercise of the same duration
The data suggest that late night bright light followed by early morning exercise had an additive circadian phase-shifting effect
Summary
The circadian system is synchronized to the earth’s 24-h rotation to promote adaptation to the environment. Synchronization occurs through exposure to daily time cues (zeitgebers). malsynchronization between circadian timing and environmental demands is a common condition with numerous negative sequelae. Youngstedt, PhD disruption, and an increased risk of cancer, depression, cardiovascular, endocrine, and gastrointestinal disease, and work-related accidents [2,3,4,5]. Likewise, delayed sleep phase syndrome has been associated with sleep curtailment and a relatively high prevalence of depression [6] and obesity [7]. Improved circadian synchronization might prevent or attenuate associated health problems
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