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Stages of Sleep

As mentioned earlier, sleep is a dynamic process. There are 2 distinct states that alternate in cycles and reflect differing levels of neuronal activity. Each state is characterized by a different type of brain wave (electrical activity that is recorded with the help of electrodes placed on the skull) 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 one and a half hours.

For the purpose of analysis, a night’s sleep is divided into 3 equal time periods: sleep in the first third of the night, which comprises the highest percentage of NREM; sleep in the middle third of the night; and sleep in the last third of the night, the majority of which is REM. Awakening after a full night’s sleep is usually from REM sleep.

NREM Sleep

Stage I is a stage of light sleep and is considered a transition between wakefulness and sleep.  During this stage, the muscles begin to relax. It occurs upon falling asleep and during brief arousal periods within sleep, and usually accounts for 5-10% of total sleep time. An individual can be easily awakened during this stage.

Stage II occurs throughout the sleep period and represents 40-50% of the total sleep time. During stage II, brain waves slow down with occasional bursts of rapid waves. Eye movement stops during this stage.

In stage III, extremely slow brain waves called delta waves begin to appear. They are interspersed with smaller, faster waves. In stage IV, delta waves are the primary waves recorded from the brain. These 2 stages are distinguished from each other only by the percentage of delta activity. Together they represent up to 20% of total sleep time. Stages III and IV are called deep sleep, during which all eye and muscle movement ceases. It is difficult to wake up someone during these 2 stages. If someone is awakened during deep sleep, he does not adjust immediately and often feels groggy and disoriented for several minutes after waking up. Some children experience bedwetting, night terrors, or sleepwalking during deep sleep.

REM Sleep

REM sleep represents 20-25% of the total sleep time. REM sleep follows NREM sleep 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 in duration, while the last may exceed 60 minutes. In a normal night’s sleep, bouts of REM occur every 90 minutes.

When the person is extremely sleepy, the duration of each bout of REM sleep is very short or it may even be absent. REM sleep is usually associated with dreaming. During REM sleep, the eyeballs move rapidly, the heart rate and breathing become rapid and irregular, and the 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%. The electrical activity recorded in the brain during REM sleep is similar to that which is recorded during wakefulness.