The stages of sleep

Every night, we go through four to six sleep cycles during which we alternate between the two sleep stages Non-REM and REM.

In particular, there are five distinct stages of sleep – four of deep or non-rapid eye movement sleep (non REM) and one stage of rapid eye movement (REM) sleep. Non REM and REM sleep are as different from each other as they are for wakefulness.

There are four stages of Non REM sleep:

• Stage 1:

During this stage the eyes barely move, muscle activity slows and you drift in and out of slumber. Someone in stage 1 sleep can be awakened easily and will remember fragmented images from a pre-dream state.

• Stage 2:

Eye movements stop and brain waves become slower. There are occasional bursts of rapid brain waves called sleep spindles. To wake someone up during this stage, they will really have to be prodded.

• Stage 3:

This transitional stage ushers in the deepest sleep of the night, marked by the appearance of extremely slow brain waves called delta waves, which are mixed with smaller, faster brain waves. It is exceedingly difficult to wake someone during stage 3 and 4, which together are referred to as deep sleep.

• Stage 4:

Now there are no eye movements, the muscles are relaxed, the blood pressure is at the lowest and the heart and breathing rates are at their slowest. The brain produces delta waves almost exclusively. This is when the body repairs itself with the aid of a hormone called somatostatin, which helps maintain the health of muscles and other soft tissues.

• REM Sleep:

When we switch into REM sleep (which usually happens about 70 to 90 minutes after we fall asleep and recurs throughout the night), our breathing becomes more rapid, irregular and shallow. The heart rate increases and blood pressure rises. The synchronized brain waves characteristic of deep sleep break up and begin to look like those of wakefulness.

The hyperactivity in the brain is coupled with almost total lack of movement – earning the REM stage its nickname, ‘paradoxical sleep’. People awakened during REM sleep often describe odd dreams.