Biological cycles within our bodies that occur on an approximate 24 hour cycle are known as circadian rhythms. Circadian rhythms promote a readiness for sleep by decreasing alertness but they do not regulate sleep. Instead about every 90 minutes while asleep our body cycles through different stages in which brain activity and other physiological responses change in a predictable way.

Scientist study sleep by monitoring the brain wave activity of people while they are asleep. This is how they have determined the different stages of sleep and when a person is in each level. It works something like this, when a person is a wake and conscious, their brain will produce a pattern of beta waves. These waves are a high frequency but a low amplitude or height. As you close your eyes and get that drowsy, relaxed feeling, your brain wave activity will start to slow down and alpha waves will start to occur.

As sleep starts, the brain wave pattern becomes more irregular and slower theta waves increase. This is known as stage 1 sleep. This is a form of light sleep from which a person could be woke up rather easily. This stage normally lasts a very short time and some people do experience dreams in this state. As you fall deeper into sleep,1-2 second bursts of rapid brain wave activity, known as sleep spindles, begin to occur. This is an indication that a person is now in stage 2. The muscles are more relaxed, breathing and heart rate slow down and dreams may occur. It is harder to wake someone in this state up than in stage 1.

Sleep deepens further as you go into stage 3. This stage is noticed by a regular appearance of very slow and large delta waves. As time goes by they occur more often and when they dominate the pattern of the EEG you have reached stage 4. Your body is relaxed, and brain activity over a large portion of the brain has decreased. After about 20 minutes in stage 4 the EEG shows that you go back through stage 3 and stage 2 spending just a small amount of time in each. In about 90 minutes you will have completed a cycle of the stages going: 1-2-3-4-3-2. After this time you will enter a different stage of sleep.

This stage of sleep is known as REM sleep. Or rapid eye movement. This stage of sleep is characterized by the rapid eye movements, high arousal, and frequent dreaming. It has been found that anyone woken up during this state report dreams. Even if it is someone who says they never dream.

During REM sleep, physiological arousal may

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