Passage Three

The amount of sleep a person needs to function effectively varies considerably from individual to individual and from time to time. Newborns spend an average of 16 hours a day sleeping. Teenagers may spend as much as 10 to 11 hours asleep each night. Students in graduate school average eight hours a night. Although the amount of sleep a person needs may vary, it does appear that everyone sleeps and that sleep is crucial to the normal functioning of the body.

Have you ever noticed that there are certain times of the day when you are more alert or more tired? People seem to have an internal biological clock that regulates the sleep-wakefulness cycle. Blood pressure, heart rate, and appetite all follow circadian rhythms(昼夜节律).A circadian rhythm is a biological clock in our body that regulates the responses of our body within a time period of 24 or 25 hours. Circadian rhythms operate even when clues for day and night are removed. For example, we usually adapt our sleep patterns to the light of day and dark of night;yet researchers who have lived for months at a time in the depths of a cave have still maintained a rhythm to their behaviours. Without any environmental clues, people maintained their circadian rhythms on about a 24- to 25-hour cycle. Researchers have determined that humans have a circadian cycle of approximately 24.18 hours.

Circadian rhythms do not control our sleep cycles;the environment and the 24-hour day control our cycles. Thus, when you miss sleep, this disruption(打乱)becomes very apparent. Some travellers experience jet lag(时差反应).This occurs when their internal circadian rhythms do not match the real clock time. What do you do to cure jet lag? It usually takes about one day for each hour of time change to return to your normal circadian clock.