Many body functions vary cyclically with a period of about a day; they have a circadian rhythm. Functions regulated in this way include sleep/wakefulness, core temperature, and the secretion of anterior pituitary hormones. Humans isolated from all external time cues show intrinsic circadian rhythms with a period of about 25 hours initially. This decoupling of circadian rhythms from the normal 24-hour period is called free running and shows that there exist intrinsic circadian clocks that are usually entrained by environmental cues called zeitgebers (German “time-giver”). Zeitgebers include light, exercise, social interactions, and work schedules. Light is the strongest. A powerful light pulse given during subjective night, produces shifts in the circadian rhythm. In humans with normal sleep patterns the nadir in core temperature occurs at about 5 a.m. A light pulse 300given during the night before this time causes circadian rhythms to be delayed (phase delay) whereas a light pulse after this time causes phase advance.