Sleep in animals: a state of adaptive inactivity

In most adult animals sleep is incompatible with mating and feeding. Many researchers view animals as being highly vulnerable to predation during sleep. Why do animals devote from 2 to 20 hours of the day to sleep, in what appears to be a nonproductive state? Why has evolution preserved this state? The presence of sleep in nearly all animals and the enormous variation in sleep time across species are best explained as adaptations to ecologic and energy demands. Sleep is not a maladaptive state that needs to be explained by undiscovered functions (which nevertheless undoubtedly exist). Rather, the major function of sleep is to increase behavioral efficiency. Greater waking activity does not necessarily lead to increased numbers of viable offspring and, hence, genetic success. Rather, genetic success is closely linked to the efficient use of resources and to the avoidance of risk. Thus, inactivity can reduce predation and injury. It also reduces brain and body energy consumption. As often stated, energy conservation is not a sufficient explanation for sleep, because the energy saved in a night’s sleep in humans is only equivalent to that contained in a slice of bread. In the wild, however, most animals are hungry and are seeking food most of the time they are awake. If ample food is available, the population of a species quickly expands until faced again with food scarcity, a phenomenon that is illustrated by the great increase in the human population. The ability of sleep to conserve energy when food is scarce constitutes a major survival benefit even if only a small amount of energy is saved. Conversely, if food is available but is time-consuming to acquire, it is highly advantageous for animals to be able to reduce sleep time without behavioral impairment. Similarly, it is highly advantageous to reduce or eliminate sleep to allow migration and to respond to certain other needs. Several examples of extended periods of elimination or substantial sleep reduction without rebound have recently been documented and appear to be strongly linked to species success. Many researchers have assumed that predation risk is increased during sleep—that more animals are killed per hour during sleep than during waking. However there is scant evidence to support this contention. Most animals seek safe sleeping sites, often underground, in trees or in groups that provide communal protection. Those large herbivores that cannot find safe sleeping sites appear to require smaller amounts of sleep and to sleep less deeply. Large animals that are not at risk for predation, such as big cats and bears, can sleep for long periods, often in unprotected sites, and appear to sleep deeply.