STAGE 1
This is the first, and lightest, stage of seep. Referred to as drowsiness, this is the transition period between being awake and asleep. During this period, the brain produces high amplitude theta waves. Lasting for a brief period, between 5 and 10 minutes, the individual drifts in and out of sleep. They can be easily awakened. If that occurs, the person will most likely feel as though they haven't slept at all. During this stage, the eyes move slowly. Muscle activity decreases and sudden contractions can occur. Some people report a sensation of falling.

STAGE 2
The second stage of sleep lasts about twenty minutes. During this time, the brain produces bursts of rapid, rhythmic activity called sleep spindles. The remainder of the time, brain activity begins to slow down. Body temperature decreases and the heart rate declines.

STAGE 3
Delta waves, which are extremely slow brain waves, begin in this stage, which is a transitional one between light and deep sleep. 

STAGE 4
More intense than the previous stage, Stage 4 lasts for about 30 minutes. This is very deep sleel. It can be difficult to wake one from this stage. Stage 4, which some refer to as 'delta sleep', is marked by very slow brain activity. Bed-wetting, sleepwalking and night terrors generally occur during this level, the last of the NREM stages, of sleep.

STAGE 5
This is the REM stage of sleep. Characterized by eye movement, increased breathing levels and brain activity, REM sleep is also marked by a decrease in voluntary muscle movement. During this time frame, other physiological changes occur such as erections in males. The blood pressure rises and the body loses some of its inability to regulate temperature. Dreams happen while one is in a REM sleep state. If the person is awoken while in REM sleep, he/she will remember their dreams. The majority of people experience between three and five intervals of REM sleep each night.