DAYTIME SLEEP PROPENSITY AFTER MODERATE CIRCADIAN PHASE-SHIFTS INDUCED WITH BRIGHT LIGHT EXPOSURE

Moderate circadian phase shifts were induced by 3 days of bright light exposure, without changing the habitual sleep schedule. Daytime sleep propensity was evaluated with multiple sleep latency tests (MSLT) con ducted before and after the light treatment. Phase shifts were estimat ed using the core body temperature rhythm recorded during constant rou tines. The subjects were divided into three groups according to the ti ming of the bright light exposure. Morning bright light exposure (Morn ing group) advanced the circadian phase by about 1.2 hours, evening br ight light (Evening group) delayed the circadian phase by 1.6 hours on average; whereas, bright light administered in the afternoon (Afterno on group) did not change the circadian phase. After the light treatmen t, daytime sleep latencies decreased in the Evening and Afternoon grou ps, but did not change in the Morning group. Reduced sleep latencies i n the Afternoon group probably reflect an increase in the manifest sle ep tendency induced by the protocol itself. It is suggested that, in t he presence of a high physiological sleep tendency, a moderate circadi an phase delay may increase further daytime sleep propensity, whereas a moderate circadian phase advance may help to maintain daytime sleep propensity at a lower level.