Amplitude Reduction and Phase Shifts of Melatonin, Cortisol and Other Circadian Rhythms after a Gradual Advance of Sleep and Light Exposure in Humans

Abstract

BackgroundThe phase and amplitude of rhythms in physiology and behavior are generated by circadian oscillators and entrained to the 24-h day by exposure to the light-dark cycle and feedback from the sleep-wake cycle. The extent to which the phase and amplitude of multiple rhythms are similarly affected during altered timing of light exposure and the sleep-wake cycle has not been fully characterized.Methodology/Principal FindingsWe assessed the phase and amplitude of the rhythms of melatonin, core body temperature, cortisol, alertness, performance and sleep after a perturbation of entrainment by a gradual advance of the sleep-wake schedule (10 h in 5 days) and associated light-dark cycle in 14 healthy men. The light-dark cycle consisted either of moderate intensity ‘room’ light (∼90–150 lux) or moderate light supplemented with bright light (∼10,000 lux) for 5 to 8 hours following sleep. After the advance of the sleep-wake schedule in moderate light, no significant advance of the melatonin rhythm was observed whereas, after bright light supplementation the phase advance was 8.1 h (SEM 0.7 h). Individual differences in phase shifts correlated across variables. The amplitude of the melatonin rhythm assessed under constant conditions was reduced after moderate light by 54% (17–94%) and after bright light by 52% (range 12–84%), as compared to the amplitude at baseline in the presence of a sleep-wake cycle. Individual differences in amplitude reduction of the melatonin rhythm correlated with the amplitude of body temperature, cortisol and alertness.Conclusions/SignificanceAlterations in the timing of the sleep-wake cycle and associated bright or moderate light exposure can lead to changes in phase and reduction of circadian amplitude which are consistent across multiple variables but differ between individuals. These data have implications for our understanding of circadian organization and the negative health outcomes associated with shift-work, jet-lag and exposure to artificial light.

Mars 520-d mission simulation reveals protracted crew hypokinesis and alterations of sleep duration and timing

“…Measurements of light in the crew facility revealed a spectral power distribution consistent with fluorescent lighting (SI Appendix, Fig. S6) with low intensity in the 446-to 477-nm wavelength region of the photon spectrum, which is the most potent region for synchronizing or phase-shifting circadian rhythms of sleep and waking (13,14) and for promoting sleep timing in polar darkness (15). A separate experiment sponsored by the European Space Agency involved adding blue-light exposure late (days 439-499) in the mission (SI Appendix, SI Text).…”

Seasonal induction of GABAergic excitation in the central mammalian clock

“…Furthermore, considering that GABA is a target for therapeutic treatments of sleep disorders and the role of the SCN in sleep regulating processes, the seasonal influence on GABAergic function may affect the therapeutic manipulation of this neurotransmitter system differently in summer compared with winter. Together, these findings imply that some crucial neuronal networks within the CNS are sensitive to changes in day length, or even to prolonged artificial light exposure common in modern society (28,29).…”

Artificial light-at-night – a novel lifestyle risk factor for metabolic disorder and cancer morbidity

“…MLT levels, measured as 6-SMT, have been found to be significantly reduced in night shift workers compared with counterpart day workers, and the effect correlates positively with the intensity of ALAN and the number of nights worked [221]. Changing the sleep-wake cycle and ALAN resulted in phase advance of the MLT rhythm, which also reduced the circadian amplitude of the hormone in healthy men [222]. MLT suppression by ALAN has a dose-dependent response to both irradiance level and duration of the exposure [223].…”