Physiologic Effects of Sleep
Sleep causes two major types of physiologic effects: first, effects on the nervous system itself, and second, effects on other functional systems of the body. The nervous system effects seem to be by far the more important because any person who has a transected spinal cord in the neck (and therefore has no sleep-wakefulness cycle below the transection) shows no harmful effects in the body beneath the level of transection that can be attributed directly to a sleep-wakefulness cycle.
Lack of sleep certainly does, however, affect the functions of the central nervous system. Prolonged wakefulness is often associated with progressive mal-function of the thought processes and sometimes even causes abnormal behavioral activities.
We are all familiar with the increased sluggishness of thought that occurs toward the end of a prolonged wakeful period, but in addition, a person can become irritable or even psychotic after forced wakefulness. Therefore, we can assume that sleep in multiple ways restores both normal levels of brain activity and normal “balance” among the different functions of the central nervous system. This might be likened to the “rezeroing” of electronic analog computers after pro-longed use, because computers of this type gradually lose their “baseline” of operation; it is reasonable to assume that the same effect occurs in the central nervous system because overuse of some brain areas during wakefulness could easily throw these areas out of balance with the remainder of the nervous system.
We might postulate that the principal value of sleepis to restore natural balances among the neuronal centers. The specific physiologic functions of sleepremain a mystery, and they are the subject of much research.