The sleep sequence can best be described with the aid of a hypnogram (see below) of one night's sleep recording. The hypnogram below describes the sleep sequence of a normal male subject in his twenties. Time is represented on the X-axis and sleep stages are marked on the Y-axis.
Usually, upon falling asleep, a subject goes through a stage of relaxed wakefulness characterized by alpha waves. From there, he passes through stages 1,2,3, and gradually descends into a deeper sleep within 30-45 minutes after the onset of sleep. After 70-90 minutes, a burst of REM sleep (marked in red) appear and lasts for approximately 10 minutes. The first sleep cycle is measured from the begining of sleep to the end of the first REM period. After the first REM period, the sleeper passes into the second sleep cycle through stages 2,3,4 at the conclusion of which REM sleep will again appear.
In spite of the similarities between sleep cycles, there are several differences. The duration of stage 2, shallow sleep, increases in the second cycle and the duration of the REM period is usually longer than the first, although it is not the case with the subject below where the first and second REM periods are almost equal in length. Apart from the variations in the relative duration of the sleep stages, the second cycle will not contain changes in either brain wave characteristics or the appearance of gross body movements.
The third sleep cycle which begins once the second REM period is over, comprises still less deep sleep and a longer period of light sleep. Finally,The fourth sleep cycle contains an even smaller amount of deep sleep, the greatest amount of deep sleep having occurred in stage 2. As the sleep cycles last for some ninety minutes, their number per night depends on the duration of sleep. A young person's sleep is usually composed of four or five such cycles while an older person has fewer.