The Inner Clock That Rules Our Lives
Guess what time it is---without looking at your watch! The chances are you will be right within half an hour. Or try waking up at a particular time--without an alarm clock. You'll probably be able to do that, too.
Just about everyone can do these things, with a little practice, because the human body has its own built-in clocks.
There is a complex series of interacting rhythms in nearly everything the human body does. It sleeps to a rhythm, eats to one, and even shows a rhythm in the electrical waves produced by the brain.
Three main rhythmic cycles affect body time: the daily rhythms of the revolving earth; the monthly orbit of the moon around the earth; and the yearly journey of the earth around the sun,
A Perfect Time Sense
Not only mankind is affected by these great natural fides. All animals and plants feel them; too. Sometimes a body clock can keep accurate time to the precise minute, as in the case of the bean aphid (蚜虫), which can either lay eggs or produce live young, according to natural time that when the daylight lasts longer than 14 minutes, live offspring are born to take full advantage of the extra warmth. If the day is shorter, the aphid lays eggs to hatch at a later time.
Man's time sense is both mental and physiological. He feels hungry every three to four hours. His brain cells show two-to-three-hour cycles of activity. Even dreams run in cycles. During dreaming, the eyes, even under closed eyelids, show rapid scanning movements. Measurements of this movement, during "rapid eye movement", or "REM", permit accurate timing of periods of dreaming. Based on this, man appears to dream in cycles of about 90 minutes.
Circadian Rhythm
Man's natural rhythm of life is balanced between the length of the 24-hour solar day and the lunar day, which is 50 minutes longer. For this reason it is called the circadian (昼夜的) rhythm. Under normal conditions the circadian rhythm is constantly influenced by the rising and setting of the sun. But people isolated from outside stimuli--such as prisoners kept constantly in cells without natural light--revert automatically to a longer natural cycle.
The circadian pattern seems to be controlled partly by the hypothalamus--an area in the base of the brain close to the pituitary (垂体) gland. The hypothalamus can regulate pituitary secretions, and thus, the activity of a number of other endocrine (内分泌) gland. Effects on the cortex of the adrenal (肾上腺) gland, and changes in release of an adrenal hormone called cortisol (皮质醇), appear to be especially important in certain kinds of rhythms. These mechanisms are highly adaptable.
Most humans do have contact with natural stimuli, and their slightly shorter version of the circadian rhythm is vitally important to them. Temperature, blood pressure, pulse, breathing, and hormone activity all rise and fall in time with the world's slow spin.
A Cycle for Illness
Similarly, ability, temper, and even resistance to infection are controlled by the circadian clock. This may be why some epileptics have seizures only at certain times of the day or why pregnant women get morning sickness.
Experiments have shown that mice injected with pneumonia germs at 4 a.m. survive better than those injected at any other time of day. This could be important to humans. If vulnerability to disease is rhythmic, then response to vaccination is likely to be rhythmic, too.
Circadian rhythms account for the difference between "owls", people who are wide awake at night and stay up late, and "larks", who rise early, do their best work in the morning, and go to bed early.
Lark and Owl Patterns
Both the lark people and the owl people have quite distinct rhythms, probably because of differences in the rhythm of

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