Sleep is a natural, recurring state of body and mind, characterized by decreased sleepiness, usually accompanied by muscular restriction, reduced sensory activity, increased sleepiness and the decreased ability to recall events. Some people refer to sleep as “non-sleeping.” It is different from sleep that occurs when one is sleeping. The term non-sleeping is used to indicate periods of time when a person is awake but does not sleep. The term sleep is used to refer to periods of time when the body is in a hibernation-like state, when most of its central nervous system functions are inactive. Although it may sound strange, sleep is necessary for the proper functioning of all parts of the body.

To study sleep, a person must be asleep or rested when the following physiological events take place. First, the brain releases neurochemicals such as serotonin and dopamine that regulate mood, appetite, cognition, arousal, and sleep cycles. Second, light sleep takes place. Third, the body enters the REM (rapid Eye Movement) stage.

The four stages of sleep are rapid eye movement (REM), non-Rapid Eye Movement (NREM), sleep, and wakefulness. Rapid eye movement (REM) lasts for about ninety seconds; NREM is the period of time from when the body wakes up to the time it sleeps again. In the middle of NREM, muscle relaxation and slow breathing begin. It is these muscles that are responsible for the dream, or non-sleep state. Muscle relaxation and slow breathing occur again while a person is awake, then again while sleep.

The fourth stage of sleep, known as stage REM, is the period of time between when the brain enters the REM stage and when the muscles in the chest and limbs begin to relax. At this stage, the heart rate begins to increase and the breathing rate slows. The heart rate will remain elevated for several minutes, and then it will begin to fall. If the sleeper is awake, the heart rate and breathing rate will decrease together until they are within normal limits.

What happens next is that the brain returns to its NREM sleep state. When this happens the person may remember nothing of the previous night’s activities, but they will recall that they fell asleep quickly and with a sense of well-being. REM sleep is for reorganizing memories and thoughts. This is why people think they remember things that they actually do not. Some people have described REM sleep as a sensation of floating down through a sea of memory, much like a dream.

Studies show that many of today’s sleep disorders result from an imbalance of brain chemicals. One type of disorder is REM sleep apnea, which happens when the person’s breathing becomes irregular. Another common disorder occurs in people with low levels of melatonin, which is a hormone produced by the pineal gland. People with insomnia are three times more likely than the average person to suffer from low levels of melatonin. Other studies have shown that people who suffer from depression are three times more likely to have trouble falling asleep.