Theory of sleep. Continuation
Sleep theories are often combined. Thus, both defense and instinct theories may include the concept of recovery. For example, Pavlov recognized the recovery function as part of his defense theory. Theory of energy saving and the theory of recovery can be considered as the theory of protection. An early version of adaptation theory included the concept of instinct as a mechanism of adaptation.
Theories of recovery and adaptation eventually became the principal centers of opposition. The reasons for this are quite clear: each of the two theories fits well with certain areas of sleep phenomena. Recovery theory is consistent with the most important consequences of sleep deprivation: when a person or animal is deprived of sleep, negative effects occur, and when they get enough sleep, these effects decrease. The adaptation theory is consistent with a wide range of data on animal sleep, reflecting the relationship of time schedules and total sleep duration with the evolutionary pressures of the environment. For example, grazing herd animals that are under heavy pressure from predators tend to sleep in short periods, interspersed with awakenings, with the total duration of their sleep being only about 4 hours per day. Gorillas, on the other hand, have little to no pressure from predators and have a limited need to search for food, sleeping for 14 hours a day.
Both of these approaches have encountered difficulties in explaining the empirical material. According to the recovery model, there should be a direct relationship between waking time and its consequences. However, it turned out that the increase in the effects of sleep deprivation is not linear, but wave-like. When subjects are deprived of sleep for two nights, they perform better on various tasks on the third day than on the second night. Sleep time should be directly related to recovery time. However, some animals take just 4 hours of sleep to restore the energy consumed in 20 hours of wakefulness, while others require at least 18 hours of sleep a day. Intraspecific individual differences in sleep patterns reveal the shortest recovery time for the longest periods of wakefulness in every 24 hours. It is also known from the study of displaced sleep, for example, due to the transfer of people to another work shift, that sleep and drowsiness are affected by the time of day. On the other hand, proponents of adaptation theories did not offer any explanation for the effects of sleep deprivation at all, and faced an unexpected question, namely, why the animal simply "does not stop the behavior" or does not rest instead of sleeping.
Both theoretical positions under consideration have experienced some difficulties in empirically justifying the underlying mechanisms. Since the very first systematic studies of sleep, attempts have been made to find a "toxin" or substance of "exhaustion" that naturally changes during wakefulness and reveals the opposite change during sleep. At the moment, it has not been possible to detect such a substance, which, moreover, would have a strictly defined line of change depending on time. Adaptation theories have to rely on an instinctive mechanism that is not strictly defined.
Since the 1960s, there has been an increase in research on sleep chronology or time charts. From experiments conducted in a time-free environment and the study of the consequences of shifting sleep time in a 24 — hour cycle (for example, in connection with switching to another work shift), it became obvious that sleep is a synchronous system. Apparently, sleep can be considered as an endogenously synchronized biological rhythm, organized on a 24-hour or circadian (Latin circa — about + dies — day) basis. For proponents of adaptation theory, it is becoming increasingly clear that the mechanism of endogenous biological rhythm could be the explanatory mechanism for choosing the appropriate sleep time.