Time-of-day-dependent effects of bright light exposure on human psychophysiology: comparison of daytime and nighttime exposure

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