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Byline: Domien G. M. Beersma (Department of Chronobiology, University of Groningen, The Netherlands, d.g.m.beersma@rug.nl); Bram A. D. van Bunnik (Department of Chronobiology, University of Groningen, The Netherlands); Roelof A. Hut [...]

Bright light can influence human psychophysiology instantaneously by inducing endocrine (suppression of melatonin, increasing cortisol levels), other physiological changes (enhancement of core body temperature), and psychological changes (reduction of sleepiness, increase of alertness). Its broad range of action is reflected in the wide field of applications, ranging from optimizing a work environment to treating depressed patients. For optimally applying bright light and understanding its mechanism, it is crucial to know whether its effects depend on the time of day. In this paper, we report the effects of bright light given at two different times of day on psychological and physiological parameters. Twenty-four subjects participated in two experiments (n = 12 each). All subjects were nonsmoking, healthy young males (18-30 yr). In both experiments, subjects were exposed to either bright light (5,000 lux) or dim light < 10 lux (control condition) either between 12:00 P.M. and 4:00 P.M. (experiment A) or between midnight and 4:00 A.M. (experiment B). Hourly measurements included salivary cortisol concentrations, electrocardiogram, sleepiness (Karolinska Sleepiness Scale), fatigue, and energy ratings (Visual Analog Scale). Core body temperature was measured continuously throughout the experiments. Bright light had a time-dependent effect on heart rate and core body temperature; i.e., bright light exposure at night, but not in daytime, increased heart rate and enhanced core body temperature. It had no significant effect at all on cortisol. The effect of bright light on the psychological variables was time independent, since nighttime and daytime bright light reduced sleepiness and fatigue significantly and similarly. sleepiness; core body temperature; cortisol; heart rate

Abstract The mammalian retina contains both visual and circadian photoreceptors. In humans, nocturnal stimulation of the latter receptors leads to melatonin suppression, which might cause reduced nighttime sleepiness. Melatonin suppression [...]

Research has been conducted on the reproduction timing. The effect of this timing on the second brood incidence and parent survival in European coots has been investigated and the results indicate that adult survival does not depend on the season.

European ground squirrels were subjected to sleep deprivation during the euthermic phase of arousal episodes varying both the timing and duration of the deprivation to examine whether the effects of sleep deprivation on subsequent sleep is similar to the effect of prior torpor. Results indicate that sleep regulation functions in a normal way during arousal periods. Moreover, it also shows that the effects of prior torpor on the electroencephalogram are qualitatively different from effects of euthermic sleep deprivation.