The only well documented effect of light exposure on endocrine function is the suppression of nocturnal melatonin. Bright light exposure has behavioral effects, including the alleviation of sleepiness during nocturnal sleep deprivation. The present study examines the effects of bright light on the profiles of hormones known to be affected by sleep deprivation (TSH) or involved in behavioral activation (cortisol). Eight healthy men participated each in three studies involving 36 h of continuous wakefulness. In one study, the subjects were exposed to constant dim light (baseline). In the two other studies, dim light exposure was interrupted by a 3-h period of bright light exposure either from 0500-0800 h (early morning study) or from 1300-1600 h (afternoon study). Blood samples were obtained every 15 min for 24 h to determine melatonin, cortisol, and TSH concentrations. Alertness was estimated by the number of lapses on two computerized vigilance-sensitive performance tasks. The early morning transition from dim to bright light suppressed melatonin secretion, induced an immediate, greater than 50% elevation of cortisol levels, and limited the deterioration of alertness normally associated with overnight sleep deprivation. No effect was detected on TSH profiles. Afternoon exposure to bright light did not have any effect on either hormonal or behavioral parameters. The data unambiguously demonstrate an effect of light on the corticotropic axis that is dependent on time of day.