Then, normally, as a person becomes sleepier, the body begins to slow down.
Muscles begin to relax, and eye movement slows to a roll.
Sleep cycles subsequent to the first one in a night's sleep typically feature less slow-wave sleep, as Stages 3 and 4 shorten.
Slow-wave, deep sleep is longest early in a night's sleep.
The period of non-REM sleep (NREM) is comprised of Stages 1–4 and lasts from 90 to 120 minutes, each stage lasting anywhere from 5 to 15 minutes.
Surprisingly, however, Stages 2 and 3 repeat backwards before REM sleep is attained.
The waking stage is referred to as relaxed wakefulness, because this is the stage in which the body prepares for sleep.
All people fall asleep with tense muscles, their eyes moving erratically.
The eyes are closed during Stage 1 sleep, but if aroused from it, a person may feel as if he or she has not slept. Stage 2 is a period of light sleep during which polysomnographic readings show intermittent peaks and valleys, or positive and negative waves.
These waves indicate spontaneous periods of muscle tone mixed with periods of muscle relaxation.