Resetting the Melatonin Rhythm with Light in Humans

Abstract

The endogenous circadian rhythm of melatonin in humans provides information regarding the resetting response of the human circadian timing system to changes in the light-dark (LD) cycle. Alterations in the LD cycle have both acute and chronic effects on the observed melatonin rhythm. Investigations to date have firmly established that the melatonin rhythm can be reentrained following an inversion of the LD cycle. Exposure to bright light and darkness given over a series of days can rapidly induce large-magnitude phase shifts of the melatonin rhythm. Even single pulses of bright light can shift the timing of the melatonin rhythm. Recent data have demonstrated that lower light intensities than originally believed are capable of resetting the melatonin rhythm and that stimulation of photopically sensitive photoreceptors (i.e., cones) is sufficient to reset the endogenous circadian melatonin rhythm. In addition to phase resetting, exposure to light of critical timing, strength, and duration can attenuate the amplitude of the endogenous circadian rhythm of melatonin. Measurement of melatonin throughout resetting trials provides a dynamic view of the resetting response of the human circadian pacemaker to light. Future studies of the melatonin rhythm in humans may further characterize the resetting response of the human circadian timing system to light.

Melatonin secretion in a strictly subterranean mammal, the Damaraland mole‐rat (Cryptomys damarensis)

“…The absence of light underground suggests that subterranean mammals are normally deprived of the most ubiquitous zeitgeber to time life-history events, namely photoperiod. However, it has been suggested that brief exposure to light can entrain melatonin secretion in subterranean rodents (Rado et al, 1993;Reiter, Reiter et al, 1994), as it does in some other species (Reiter, 1993;Shanahan et al, 1997). In this study, melatonin secretion in a strictly subterranean rodent C. damarensis was found to be both diurnal and circadian, but relatively insensitive to acute exposure to light and dark.…”

A Short Nap and Natural Bright Light Exposure Improve Positive Mood Status

“…Given that light temperature rather than its intensity has received considerable interest 39,40) and that the color temperature of natural light differs between morning and evening, the next research question is the sensitivity to natural light exposure at different times of day (i.e., morning, midday, evening). Although many studies have compared light sensitivity, which varies at different times of day and different color temperatures [41][42][43] , to the best of our knowledge, few studies have investigated its immediate effects on arousal level or mood status. Light sensitivity research may contribute to architectural lighting design and the effective use of natural light in developing optimal economical and ecological occupational environments.…”

Experiments on the effects of a continuous 16.7 Hz magnetic field on melatonin secretion, core body temperature, and heart rates in humans

“…The highly reproducible diurnal pattern of melatonin synthesis in the same individual and the great interindividual differences [Shanahan and Czeisler, 1991;Graham et al, 1996a,b;Shanahan et al, 1997] required within-subject comparisons between an exposure session and a control with identical experimental procedures.…”