The Four Stages of Sleep

The Four Stages of Sleep

  • NREM stage 1: This is a stage between sleep and wakefulness. The muscles are active, and the eyes roll slowly, opening and closing moderately.
  • NREM stage 3: Formerly divided into stages 3 and 4, this stage is called slow-wave sleep (SWS). SWS is initiated in the preoptic area and consists of delta activity, high amplitude waves at less than 3.5 Hz. The sleeper is less responsive to the environment; many environmental stimuli no longer produce any reactions.
  • REM: The sleeper now enters rapid eye movement (REM) where most muscles are paralyzed. REM sleep is turned on by acetylcholine secretion and is inhibited by neurons that secrete serotonin. This level is also referred to as paradoxical sleep because the sleeper, although exhibiting EEG waves similar to a waking state, is harder to arouse than at any other sleep stage. Vital signs indicate arousal and oxygen consumption by the brain is higher than when the sleeper is awake.  An adult reaches REM approximately every 90 minutes, with the latter half of sleep being more dominated by this stage. REM sleep occurs as a person returns to stage 1 from a deep sleep. The function of REM sleep is uncertain but a lack of it impairs the ability to learn complex tasks. One approach to understanding the role of sleep is to study the deprivation of it. During this period, the EEG pattern returns to high-frequency waves that look similar to the waves produced while the person is awake. 

Wikepedia has a great explanation:

http://en.wikipedia.org/wiki/Sleep

Sleep diagram