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. |