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Day and night alternate over 24 hours due to the rotation of the planet, and the start and length of daylight varies with the seasons.

So internal biological clocks (controllers) evolved for controlling activities related to the environment such as those of cold-blooded animals which need to maintain their body temperature by warming themselves in the sun. Reptiles are cold-blooded animals warmed by the daylight sun and conserve energy by restricting activities when it is dark. And the biological clock which controls their activity-rest cycle is located within the eye.

But about 180 million years ago, warm-blooded mammals evolved from their cold-blooded reptilian ancestors by developing the ability to maintain a constant body temperature by biological processes. This freed them from depending on daylight and the weather for survival. Deep sleep appeared at the same time.

The earlier mammals were reproducing themselves by hatching their young out of eggs. But about 180 to 130 million years ago, many mammals evolved into giving birth directly from the womb, their young being born alive after having been developed for a considerable period within the womb. Their young have to grow and learn much for a long time before they can survive independently, for many years in the case of human beings. The human brain now has much greater learning capacity.

In mammals, information about light and darkness is transmitted from the eye to a biological clock, now situated in the mammalian brain, which controls the sleep-wakefulness rhythm. Another biological clock controls the body-temperature rhythm, and these biological clocks together control the related body-temperature and sleep-wakefulness rhythms.

While the body's temperature is held at a constant level, it varies by about 0.5 deg C from a low at about 05.00 hours to a high at about 18.00 hours. It appears that we tend to go to sleep after our body temperature has began to fall and tend to wake up after it has started to rise.

"The length of the geophysical day is 24 hours. Our sleep-wakefulness rhythm (circadian rhythm) has a duration which varies from individual to individual (usually between 25 and 28 hours) but is always longer than 24 hours. And our biological rhythms are adjusted accordingly, day by day," by these internal biological clocks, to the external geophysical day, to the environment. People sleep, on average, between 6.5 and 8.5 hours.

The body-temperature clock also controls the appearance of REM sleep.

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