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Stages of Sleep

There are 2 distinct states that alternate in cycles and reflect differing levels of brain activity. Each state is characterized by a different type of brain wave activity. Sleep consists of non–rapid eye movement (NREM) and rapid eye movement (REM) sleep.  NREM is further subdivided into the following 4 stages:

  • Stage I (Light sleep)
  • Stage II
  • Stage III & IV (Deep Sleep)

The stages of NREM sleep and REM sleep cycle over and over again during a night’s sleep. Stages I, II, III, and IV are followed by REM sleep. A complete sleep cycle, from the beginning of stage I to the end of REM sleep, usually takes about 1 and 1/2 hours.

State and Stage Description % of Total Sleep Time
Non-Rapid Eye Movement (NREM) Stage 1
(Light Sleep)
  • Considered a transition between wakefulness and sleep when the muscles begin to relax
  • Occurs upon falling asleep and during brief arousal periods within sleep
  • Period when you are most likely to be awakened
5-10%
Stage II
  • Occurs throughout the sleep period
  • Brain waves slow down with occasional bursts of rapid waves
  • Eye movement stops
40-50%
Stages III & IV
(Deep Sleep)
  • Extremely slow brain waves called delta waves begin to appear and are interspersed with smaller, faster waves
  • In stage IV, delta waves are the primary brain waves
  • Difference between Stages III and IV are in the percentage of delta activity
  • All eye and muscle movement ceases
  • You are not easily awakened
  • If you are awakened, it takes time to adjust and you may feel groggy and disoriented for several minutes after waking up
Up to 20% (both stages together)
Rapid Eye Movement (REM)
  • REM follows NREM and occurs 4-5 times during a normal 8- to 9-hour sleep period
  • The first REM period of the night may be less than 10 minutes, while the last period may exceed 60 minutes
  • In a normal night’s sleep, cycles of REM occur every 90 minutes
  • If you are extremely sleepy, the duration of each cycle of REM sleep is very short or may even be absent
  • REM sleep is usually associated with dreaming: Your eyeballs move rapidly, your heart rate and breathing become rapid and irregular, and your blood pressure rises.
  • The muscles of the body are virtually paralyzed
  • The brain is highly active during REM sleep, and the overall brain metabolism may be increased by as much as 20%
Up to 20-25%
Source: eMedicine.com, Inc.
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