To redress the shortcomings of the repair and restoration theory, an alternative theory of a need to sleep has been proposed. The adaptive or evolutionary theory postulates that the need to sleep arose in the course of biological evolution as an adaptive mechanism to conserve energy during the evening hours, when it would be inefficient to search for food and other resources. Sleep, according to this view, serves a function similar to the hibernation observed in several species of mammals. These animals reduce their metabolic processes to barely detectable levels during winter to conserve energy when food resources are scarce. To do otherwise would threaten the survival of these animals. It is important to note that the adaptive theory still considers sleep to be a real need; in essence, sleep is a remnant of the human evolutionary past when human forebears did not have the convenience of twenty-four-hour supermarkets to acquire their sustenance. Humans deprived of sleep will become just as irritable and ill-tempered as a groundhog prevented from hibernating.
Several predictions have been generated from the adaptive theory, most of which have been supported by scientific observations. First, the theory predicts that predators such as large cats and bears, which obtain most of their nutrients in one large meal per day, would sleep much more than grazing animals such as cattle and horses, who must eat frequently to survive. A second prediction of the theory is that predators such as wolves and mountain lions, which have few natural enemies, would sleep more than prey such as rabbits and guinea pigs, which are at risk if they fail to maintain constant vigilance. Finally, animals such as bats, which are well protected by the environment in which they live, would sleep for relatively long periods of time. These predictions are documented by scientific observations, which provide support for the adaptive or evolutionary theory of sleep.
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