We human beings have developed special technologies, such as air conditioning and oil burners, to protect ourselves from discomfort and death by extremes of heat or cold. Our bodies have also evolved strategies for maintaining a fairly constant body temperature despite large fluctuations in the environmental temperature. One such strategy involves the unconscious brain-based rhythms that time when we sleep and wake. The timing of sleep is controlled by a brain clock designed for twin strategies: survival and energy conservation.
In the mid-nineteenth century, German physiologists recognized the link between sleep and body temperature. They established the clinically important fact that human beings normally experience daily fluctuations in core body temperature of about 1.5 degrees Fahrenheit, with a peak in the late morning or early afternoon and a trough at night—coinciding with sleep. It has since been well documented that this rhythmic change is not a reflex response to the more extreme daily fluctuations of ambient light and temperature. It is, rather, a rhythm generated by the brain, which anticipates the ambient variability and fits behavior to it.
Why does body temperature vary? And why do people sleep when body temperature is low? The answers lie in the connection between body temperature and metabolism. Our bodies constantly generate heat as well as obtain biochemical building blocks for our tissues. To sustain metabolic activity, we must obtain energy by consuming food.
Within the same organism, a high level of metabolic activity will coincide with a high body temperature. Because sleep is a state of lower metabolic activity, with a lower body temperature, it is energy conservative. Even if we cannot seek food during sleep, we do not use up much energy, because our muscles become inactive. And interestingly, we do not normally become hungry while asleep. Removing individuals temporarily from the effort to find nourishment is one of the main contributions sleep makes to the survival of mammalian species. By decreasing the amount of food that needs to be consumed, sleep makes it possible for animals to compete more successfully for limited food supplies.