Psychophysiological effects of early morning bright light exposure in young adults.

The effects of bright light on circadian rhythms in man are well documented. Nevertheless the theoretical basis and the rules for the practical utilization of light exposure as therapy need still to be better defined. The present study determined to what extent a 2-hr bright light exposure (0500-0700 h) improved the adjustment to an early rising in normal adults. Phase changes were assessed in subjective alertness, performance in several search tasks, time estimation, and a visual discrimination task, as well as in body motility, plasma cortisol concentrations, and body temperature. In comparison with a dim light exposure, the bright light resulted in increased motor activity during waking, in earlier peak of subjective alertness, and an improvement in performance speed in three out of five tasks in the morning. Cortisol and body temperature also were phase-advanced. In summary, light applied to a portion of the circadian cycle sensitive to phase advance shifts influenced rhythms with strong endogenous components (temperature and cortisol), while other rhythms with strong exogenous components were more sensitive to sleep deprivation caused by the early rising time.