2-HOUR AND 4-HOUR BRIGHT-LIGHT EXPOSURES DIFFERENTIALLY EFFECT SLEEPINESS AND PERFORMANCE THE SUBSEQUENT NIGHT

The effect of two durations of bright light upon sleepiness and perfor mance during typical night shift hours was assessed. Thirty normal, he althy young adults participated in a 2-night protocol. On the Ist nigh t subjects were exposed to bright or dim light beginning at 2400 hours , under one of the following three conditions: bright light for 4 hour s, dim light for 2 hours followed by bright light for 2 hours or dim l ight for 4 hours. Following light exposure, subjects remained awake un til 0800 hours in a dimly lit room and slept in the laboratory between 0800 and 1600 hours, during which time sleep was estimated with actig raphy. Throughout the 2nd night, the multiple sleep latency test (MSLT ), simulated assembly line task (SALT) performance, and subjective sle epiness were recorded. The single, 4-hour exposure to bright light was found to significantly increase MSLT scores and improve SALT performa nce during the early morning hours on the night following bright-light exposure. No significant effects were noted with a 2-hour exposure. T he most likely explanation for these findings is a phase delay in the circadian rhythm of sleepiness-alertness.