The time spent in NREM sleep is lightly shaded and the time spent in REM sleep is shown in black. NREM sleep always is the first to occur at the beginning of the night. From the olocket of sleep to the end of the first REM period is the first sleep cycle. Thus, the cyclic alternation of NREM and REM sleep constitutes the basic sleep cycle. The average periodicity of this cycle is ninety minutes.
Later cycles tend to be a little shorter.
As you can see, Stages 3 and 4 dominate the NREM periods in the first part of the night, but are completely absent during the later cycles. Toward the end of the night, very brief periods of wakefulness may interrupt sleep. Mostly it doesn't. Depth of sleep, when it is used in a generalized sense to describe the overall character of a portion of sleep, is an archaic concept-a holdover from the old conception of sleep as simply a diminution of waking activity. But the facts are that there is no generalized depth of sleep whereby all activities increase or decrease in unison, as described below.