The Stages of Sleep

When you close your eyes and fall off to sleep, do you have any idea what's going on inside your body? Although you are at rest, not every part of your body is resting. The brain in particular is sending out all sorts of signals, many of which are controlling the various stages of sleep you'll be undergoing. Eye movements and muscle activity change throughout the various stages of sleep too.

Five stages of sleep have been identified, and on a good night's sleep you pass through all of them, several times. Although there are 5 distinct stages, the first four stages are grouped together and classified as the state of non-rapid eye movement or NREM. The fifth stage is considered the state of rapid eye movement or the REM stage.

The different states of NREM and REM are distinguished by the changes in brain wave activity that takes place. REM is also characterized by irregular but rapid breathing and heartbeat, a rise in blood pressure, increased brain activity and virtually no muscle activity.

NREM and REM sleep take place throughout the sleep period. The sleep that takes place during the first third of a night consists mostly of NREM while the sleep during the last third is mostly REM. Usually you will awaken from a REM stage.

Stage 1 is a transitional period. It consists of light sleep from which you can easily be awakened. During stage 1, the cycle of falling asleep and waking can repeat several times. During this stage you feel drowsy and begin to lose control over your muscles and your eye movements slow. During this stage, hypnic myoclonia, a sudden contraction or jerking of muscles, can happen.

Stage 2 occupies nearly half of the sleep period. During this stage the eyes stop moving, brain waves slow, body temperature drops and heart beat slows. Occasionally there are short bursts of eye movement.

Stages 3 and 4 are the stages during which the body is in a deep sleep. These 2 sleep stages are characterized by the presence of delta waves - brain waves that are extremely slow. Eyes and muscles are completely still during stages 3 and 4.

The REM stage takes place throughout the sleep period. REM always follows a period of NREM sleep and accounts for about a quarter of the total sleep period. The first REM occurs at the end of Stage 1 and lasts about 10 minutes. The final REM stage lasts longer, about an hour. If you sleep 8 or 9 hours, you'll typically experience REM sleep 4 or 5 times. You're able to dream during REM, too.

These sleep stages repeat throughout the sleep period. These repetitions are known as sleep cycles. Several sleep cycles take place during a single sleep period. As the sleep cycles progress through a sleep period, the amount of REM sleep increases. Keeping these sleep cycles from becoming disrupted is what results in better-quality sleep.