Refer to figure.
SLEEP

Sleep is essential to human well-being. During a sleep period the body is not only recuperating from the physical activity of the day but it is also carrying out essential organization of the mental processes. 

Sleep stages. The stages of sleep are classified into 4 stages:

• Stage 1. The sleeper is in a very light sleep. It is a transitional phase between waking and sleeping; if woken at this stage, the sleeper may claim that he/she has not even been asleep. In early sleep we pass through about 10 minutes of Stage 1 before moving to the deeper Stage 2.
• Stage 2. In early sleep we spend about 20 minutes in stage 2 before moving on to the deeper Stage 3 and 4. About 50% of a normal spent is spent in Stage 2.
• Stage 3 and 4. During Stage 3 and 4 sleep the brain is semi-active emitting long slow waves, it is commonly referred to as “Slow Wave” or Orthodox sleep. The eyes are still behind the eyelids and the muscles are relaxed. Slow wave sleep refreshes the body and is necessary for tissue restoration. After strenuous physical activity, the body will require more slow wave sleep.

Rapid Eye Movement (REM) sleep

Superimposed on the above 4 stages is REM (or Paradoxical sleep). In this phase the brain is active. Rapid eye movement behind the eyelids are detected and muscles twitch. It is complex, bizarre and emotionally colored dreams take place. REM sleep refreshes the brain. It strengthens and organizes the memory. After a period of learning new tasks or procedures REM sleep will increase.