DEMONSTRATION OF RAPID LIGHT-INDUCED ADVANCES AND DELAYS OF THE HUMANCIRCADIAN CLOCK USING HORMONAL PHASE MARKERS

To determine the magnitude and direction of phase shifts of human circ adian rhythms occurring within 1 day after a single exposure to bright light, plasma thyrotropin, melatonin, and cortisol levels and body te mperature were monitored for 38 h in 17 men who were each studied two times, once during continuous dim light conditions and once with light exposure. After a period of entrainment to a fixed sleep-wake cycle, a 3-h light pulse (5,000 lux) was presented under constant routine con ditions, and the resultant phase shifts were measured, also under cons tant routine conditions, on the 1st day after pulse presentation. The phase shifts in response to light occurred within 24 h and were in the delaying direction for most of the nocturnal period, with the crossov er to phase advances occurring similar to 1 h after the temperature mi nimum. Phase shifts averaged 1 h, with delays being larger than advanc es, and were achieved without significant changes in rhythm amplitude. The immediate response of the human circadian clock to a single 3-h l ight pulse is thus characteristic of ''type 1'' resetting.