Bright light affects alertness and performance rhythms during a 24-h constant routine.

The aim of the present study was to assess the stimulating effects of bright light (BL) on subjective and objective alertness. Eight subjects were exposed to either bright light or dim light (DL) during a 24-h constant routine (0900-0900). Bright light failed to modify either the 24-h course or the level of body temperature. Compared to DL, BL delayed the circadian trough of motor activity by 2 h. During the night, relative to the dim-light condition, BL significantly increased subjective and objective (EEG test) alertness and improved performances. Thus, BL exposure partly counteracted the effects of sleep deprivation and/or the circadian trough on alertness and performances. During the day, BL only improved the mood and motivation levels. However, the time course of mood and motivation was not affected by the BL exposure, a nocturnal circadian trough occurring at 0630 in both light conditions.