Stages of Sleep

Posted on July 22, 2009 by brett — No Comments ↓

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.