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) conducted before and after the light treatment. Phase shifts were estimated using the core body temperature rhythm recorded during constant routines. The subjects were divided into three groups according to the timing of the bright light exposure. Morning bright light exposure (Morning group) advanced the circadian phase by about 1.2 hours, evening bright light (Evening group) delayed the circadian phase by 1.6 hours on average; whereas, bright light administered in the afternoon (Afternoon group) did not change the circadian phase. After the light treatment, daytime sleep latencies decreased in the Evening and Afternoon groups, but did not change in the Morning group. Reduced sleep latencies in the Afternoon group probably reflect an increase in the manifest sleep tendency induced by the protocol itself. It is suggested that, in the presence of a high physiological sleep tendency, a moderate circadian 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.