The function of normal sleep is divided into 5 stages which can be put into two classifications, either NREM sleep or REM sleep. These stages can be easily distinguished on an EEG.
The five stages of sleep are distinctive and can be identified through the use of a limited EEG. An EEG is performed during a sleep study to measure the continuous brain wave patterns otherwise known as the electrical activity of the brain. This activity is recorded and the 5 stages are categorized as either NREM or REM sleep.
Normally when sleeping you experience four to six cycles of NREM each night. These cycles are followed by very brief episodes of REM sleep. A complete cycle lasts only about 90 minutes and then repeats itself. As the sleep progresses through the night the periods of NREM shorten and the REM cycles lengthen. On the average adults spend only about 25% of the night in REM sleep cycle, which may decrease as they age.
Stages of Sleep – NREM Sleep
NREM is an abbreviation for Non Rapid Eye Movement. NREM sleep consists of three stages of sleep and each has electrical brain wave patterns that can be distinguished by the EEG. The stages of sleep in this cycle include N1, N2 and N3.
N1 stage of sleep is the stage at which we first begin to transition into sleep. This only lasts a few minutes before we move into our baseline sleep. Baseline sleep is N2 or stage two sleep. This stage occupies about 50 to 65% of the time we sleep. This stage of sleep will move into stage 3 and 4 sleep after about 15 or 20 minutes. Stage 3 and stage 4 sleep are referred to as delta sleep because of the slow and very high voltage of electrical brain waves during these periods of sleep. Delta sleep is a deep sleep like a coma, but reversible. Delta sleep is the deepest of all the sleep cycles, it is during this sleep that the body rests and recuperates. It is commonly thought that this occurs during REM sleep. It is this stage of sleep that the sleep deprived person’s brain needs to recover. Delta sleep comprises approximately 40% of children’s sleep time and it is this sleep that can make a child dead asleep and so difficult to wake up during the night.
As sleep continues the respiration and heart rate begin to slow down. The body is almost immobile, but not quite. Very quickly we after about 20 to 30 minutes of delta sleep we will lighten our sleep to stage 2 and then all of a sudden our brain wave pattern will enter REM or paradoxical sleep. At the same time as this change into REM sleep occurs our heart rate and respiratory rate will increase substantially and during this phase we have no control over our muscles.
The length of time we spend in each stage of sleep can be changed due to conditions such as sleep apnea or insomnia. These two conditions for example can lengthen stage 2 sleep and delay or prevent us from transitioning to the other stages of sleep.
Stages of Sleep – REM Sleep
REM is the medical abbreviation for Rapid Eye Movement sleep and it occurs several times during a night of sleep. It comprises the shortest amount of time during our sleep. This is a deep stage of sleep and during this stage there is very intense activity in certain parts of the brain.
During REM sleep your eyes move rapidly in different directions. You have no motor function during REM sleep other than that of your eyes and your diaphragm which causes you to breathe. It is in this stage of sleep that dreams occur as the brain begins to hallucinate. The loss of muscle use during this stage of sleep may help to conserve energy and protect you by preventing you to act out your dream. REM sleep causes changes in your blood pressure, heart rate and respiratory rate. In addition, blood flow is increased to the brain, penis and clitoris which results in engorgement of these appendages.
This period of REM sleep will last about 10 to 20 minutes before we return to stage 2 sleep and begin the sleep cycle again. Basically during REM sleep we are a very highly activated brain within a body that is paralyzed.
This pattern will repeat itself throughout the complete night of sleep. As the sleep continues, the periods of time that alternate between delta sleep and paradoxical sleep change in duration. We begin to lose time of delta sleep which is replaced by periods of paradoxical sleep. By the final sleep cycle we are spending half of the sleep time in stage 2 sleep and half in REM sleep.