Circabidian rhythm: its appearance and disappearance in association with a bright light pulse

A shift from circadian to circabidian periods or vice versa was observed in the rhythm of sleep and wakefulness under temporal isolation. The shift occurred in two subjects, 3 times in association with a single bright light pulse applied, 4 times in all. The finding suggests that the bright light pulse has an additional effect to the phase shift on the human circadian system.