"The natural periodic loss of consciousness during which the powers of the body are restored."
Encyclopedia Britannica
Sleep is very difficult to define- it's very complex and has many aspects even today's scientists don't understand. To someone watching someone else sleep, it looks like a coma. But, sleep is very easy to wake from and is distinct from comas and hibernation.
In sleep, the eyes are usually closed, the muscles relaxed and the person is lying down. In most vertebrate, there are stages of sleep- different stages of activity in the eyes, activity of the muscles, and activity of the mind. Scientists are unsure about the usefulness and purposes of each stage and sleep in general. Some people say its' about using less energy or recuperating from a days' work. No one is for sure now.
Different people need different amounts of sleep. Children and infants can sleep twelve or more hours each day in short naps. Teens and adults usually sleep about 7 and a half hours a day, with a range of about six to ten. Age usually determines the amounts of sleep someone needs. The majority of your life will be long, uninterrupted sleep. But as you get older, you revert to your infantile system of sleeping in short naps.
Sleep is basically composed of two types of sleep- REM and NREM. REM is Rapid Eye Movement, which composes about 20%-30% of sleep. NREM is Non-Rapid Eye Movement, which is the rest of sleep. NREM is divided up into four separate stages.
Sleep begins with Stage 1. In this stage, you're right between being awake and being asleep, and it's very easy be awakened. Your muscles slowly begin to relax in this stage. In Stage 2, breathing and heart rate slow. Breathing and heart rate continue to fall and body temperature falls in Stage 3. In Stage 4 the muscles are completely relaxed. This is the deepest kind
of sleep. Sleep walking, sleep talking, and dreaming occurs during these four stages.After falling asleep, you move from NREM Stage 1 to Stage 4, then backwards back to Stage 1. Then, you enter a 10-15 minute long REM stage. Afterwards, the cycle restarts. This long cycle repeats as long as you're asleep, but each time the REM stage gets a little longer.
Dreaming occurs primarily during the NREM type of sleep. Dreams are a mostly unknown branch of science. They are essentially hallucinations or illusions in your mind. Some psychological experts say dreams are an accumulation of of suppressed feeling such as anger or guilt. Dreaming frequency and length seems to be regular. While many dream-like states have been found in animals and there have been advances in the knowledge of dreams, scientists still know very little about their purpose or their cause. Scientists also know very little about their effects on sleep and sleep deprivation.
