With a number of modern electron physiological experiments nocturnal sleep has demonstrated that it is composed of different phases that are repeated in cycles.
There have been two main phases of sleep: Orthodox (slow) sleep and paradoxical (rapid) sleep. This is where they observed differences in the behavior of sleeping, the depth of sleep, movement of the eyeball and others.
Stages of the orthodox (slow) sleep. Orthodox sleep consists of four consecutive stages.
The first stage is called, nap. During this sleep we sleep away gradually detached from the outside world. Eyeballs are swirling making slow movements and in electroencephalography (EEG), which provides information on the work of the brain during sleep, dominated by alpha waves of small amplitude.
The second stage is a light sleep. In electroencephalography appear sleep spindles - bursts of fluctuations with frequency 12-14 per second.
The third stage has an average depth of sleep. This is the longest in the first third of the night shows a electroencephalography slow increase of delta waves.
The fourth stage is a deep sleep. Electroencephalography shows numerous delta waves with a long duration.
Paradoxical (faster) sleep. During that sleep electroencephalography observed fast waves of low amplitude that is why ti is called REM sleep, RAPID EYE MOVEMENT.
There are, rapid movements of the eyeball. During this sleep we dream our dreams. REM sleep occurs 45-60-90 minutes after the beginning of sleep and repeated after each cycle of the orthodox (slow) sleep. It is deeper than the slow sleep.