The Functions of Sleep
>Why is sleep so necessary? What functions does sleep serve? Why does sleep incorporate such diverse features as SWS and REM sleep? Currently, there are no firm answers to these questions, although accumulating research has led to exciting speculation about changes in brain chemistry during waking and sleep. Explanations of why sleep occurs have focused generally on such concepts as rest and restoration, energy conservation, and memory consolidation. There also are ecological arguments regarding temporal niches for species to maximize waking productivity (e.g., food acquisition) and sleep-time safety.
The idea that sleep has a role in energy conservation is particularly compelling, at least with regard to NREM sleep. From this point of view, sleep can be seen as helping to reduce energy consumption and perhaps resupplying brain energy constituents depleted during waking. Conceivably, the accumulation or depletion of key substances could contribute directly to the experience of homeostatic sleepiness. Other substances may be recycled. Glucose and oxygen use are decreased markedly during NREM sleep. While this alone may be advantageous, NREM sleep also may offer the opportunity for the repletion of energy stores. It has been suggested that brain glycogen is accumulated during NREM sleep. Adenosine also has been a focus of speculation regarding an energy-management theory of sleep. Clearly, adenosine has a vital energy role throughout the body by way of the ATP cycle. Adenosine antagonists, such as caffeine and theophylline, obviously inhibit sleep. Conversely, adenosine agonists seem to enhance sleep.
There has been widespread speculation about the functions of REM sleep, especially in relation to dreaming. There are multiple psychoanalytic and other psychodynamic explanations of why we dream. The unique physiological characteristics of REM sleep have stimulated other speculation. Since REM sleep has similarities with activity of the waking brain, it may represent an alerting mechanism. Perhaps REM sleep facilitates awakening in the morning. This would be consistent with the observation of little sleep inertia resulting from REM sleep awakenings and the fact that most REM sleep occurs toward the end of a normal night. REM sleep also has been seen as critical in brain development because fetuses, as well as newborns, spend great amounts of time in REM sleep.
Source: David N. Neubauer, “Understanding Sleeplessness: Perspectives on Insomnia,” The Johns Hopkins University Press, Baltimore 2003
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