THE PHYSIOLOGY OF SLEEP
At one time, sleep was considered a passive process. The revolutionary discovery of the ascending reticular activating system by Moruzzi and Magoun changed this theory. Today sleep is considered an active process, regulated by the reticular activating system. Increase in the activity brings about wakefulness, and diminution of the activity brings about the two states of sleep. This theory was substantiated by the finding that sleeping animals can be awakened through stimulation of electrodes implanted in the midbrain reticular formation.
STATES OF SLEEP
Three life states can be distinguished: wakefulness, slow-wave sleep (SWS) (non-rapid eye movement - NREM sleep), and paradoxic sleep (PS) (rapid-eye movement - REM sleep).
In wakefulness, one is fully self-conscious and also conscious of the environment; any external stimuli results in maximal response from an individual. On the other hand, sleep is a period of inertia and low responsiveness. It can be terminated by insistent external stimuli.
The discovery of the electroencephalogram provided evidence that the brain displays continuous electrical activity. Electroencephalographic (EEG) studies by Aserinsky and Kleitman led to the discovery of the two kinds of sleep. Dement and Kleitman subdivided slow-wave sleep into four stages, during which rapid eye movements are absent. The fifth stage is the paradoxic sleep characterized by rapid eye movement. Electroencephalographic studies established a cyclic pattern of states of sleep.
Each state of sleep is characterized by different EEG patterns. Wakefulness is characterized by low-voltage fast activity of the EEG, high muscle activity, and numerous rapid eye movements indicating intensive interaction with the environment. In wakefulness, the electromyographic (EMG) recording shows a high level of muscular activity in the head and neck areas. The electrooculogram (EOG) shows frequent eye movements. When sleep overtakes wakefulness, the transition is gradual. At first, drowsiness (stage 1 sleep) sets in, characterized by 8 to 12 cycles per second (cps) alpha-rhythm in the EEG and by muscular tone relaxation and slow eyeball oscillation in the EOG. After a few minutes, stage 2 is reached, which involves definite sleep characterized by a 12- to 15-cps EEG pattern. From 45 to 50% of the total sleep period is spent in stage 2 sleep. As stage 3 sleep approaches, the EEG voltage continues to increase and the frequency continues to decrease. A gradual change into stage 4 sleep characterized by delta waves (high amplitude, slow wave) is observed. After the NREM sleep has been completed (stage 1 to stage 4 combined), a sudden burst of activities begins as REM sleep takes over, characterized by rapid eye movements and a mixed-frequency EEG pattern.
THE SLEEP CYCLE
Under normal circumstances, normal young adults at night display a regular pattern of sleep, 20 to 25% of which is REM sleep and 75 to 80% of which is NREM sleep. NREM always precedes REM sleep. After about 90 minutes of NREM sleep, the first REM sleep occurs, with a mean duration of about 20 minutes. Afterwards, REM periods occur cyclically at intervals of about 90 minutes. Throughout a night, four or five periods of dreaming (REM sleep) take place, accounting for about 20% of the total sleep time.
The effect of dream deprivation was extensively studied by Dement. If the subject was awakened every time the EOG and EEG indicated that dreaming has just begun, he or she became selectively deprived of REM sleep; because every time he or she fell asleep again, the sleep cycle began with NREM sleep.
As a result of selective REM sleep deprivation, REM sleep appears at shorter and shorter intervals, and the subject has to be awakened more often. As a result, a pressure for REM sleep builds up.
This REM sleep deprivation is also manifested by the so-called rebound effect. When the subject previously deprived of REM sleep is allowed to sleep, the REM sleep period is longer than normal. Most hypnotics and addicting drugs suppress REM sleep, and after their withdrawal, a rebound excess of REM sleep occurs.
The need for NREM sleep has also been demonstrated. Subjects deprived of all sleep spend more time in NREM sleep.
In conclusion, the main point to take away from this article is that many autonomic, physiologic and biochemical changes are associated with wakefulness, slow-wave and paradoxic sleep.