The foundations of modern physiology and psychology were laid in 1953, when Aserinsky and Kleitman observed that sleeping people actively move their eyes during certain periods of time as if searching for something. In 1958 Diment showed that the chart of the electrical activity of some parts of the brain during these phases of sleep is similar to the one showing the brain electrical activity of a problem solving person who is awake. These phases of sleep, which may occur 4-5 times during the night, are called “paradoxical”/REM (rapid eye movement), (or “quick”, “dreaming”), as while the person is asleep, the brain is still physiologically “awake” and the eyes move quickly. Another phase of sleep is called “Orthodox”/NREM (non-rapid eye movement), (or “slow”, “non-dreaming”), which in its turn consists of four stages. The first is the stage of slight sleepiness or slumber. It is the superficial phase of sleep. The second is the average level of sleep. If nothing disturbs the sleep, the second stage is followed by the third stage of deep sleep.
The fourth stage is the deepest level of sleep. During this stage slow electrical activity prevails. The fourth stage of sleep, if generally observed, lasts too short in normal conditions and occurs only at the beginning 40-50 minutes after falling asleep. During the first hour of sleep the stage of average intensity prevails. The fourth stage is typical of people who relax after hard physical work, while people doing mental work hardly ever experience this orthodox stage.
As mentioned above four stages together form the orthodox or “slow” phase of night sleep. From time to time (4-5 time during eight hours’ sleep) orthodox phase is followed by paradoxical phase. An orthodox phase followed by a paradoxical phase together form one cycle of sleep.
During a normal night sleep of a forty-year-old person the first stage of orthodox sleep covers 10% of the total duration of sleep, the second covers 53%, the third covers 5%, the fourth covers 10%, and the paradoxical sleep covers 22-25%. These indicators are somewhat different at different ages. As we grow older our sleep becomes more and more superficial, the duration of the deep sleep reduces and even completely disappears at very old ages.
At the paradoxical stage of sleep other physiological functions are similar to the ones experienced when awake. The rate of heart beats is 45-100 per minute, and breathing becomes irregular.
If one is woken up from orthodox sleep and kept awake for several minutes, and then again falls asleep, the orthodox sleep will start only after at least 30 minutes from the beginning of the sleep. One can optionally be deprived of either orthodox or paradoxical sleep. In such cases the phases of sleep they have been deprived of become more prolonged. Deprivation of any of these stages causes a feeling of having no sleep at all. The night sleep is complete only if these two stages normally follow each other 4-5 times during the night.