Many people consider sleep a passive process -- probably because most the time we are motionless. Actually, sleep is an active brain process. There is no well defined “sleep” center in the brain like there is a visual center. Instead, various areas of the brainstem are responsible for producing sleep stages. In humans sleep can be studied by attaching recording electrodes to the scalp, over the chest, and over limb muscles. In other words by monitoring brain waves, heart rate, breathing, and muscle tone.
As we enter the first stages of sleep the EEG begins to slow along with our breathing and heart rate. This is termed Slow Wave Sleep (SWS). We progress through four stages of SWS (stages are judged by the amount of slow waves) several times a night. Although we may dream during these periods, the dreams are less vivid and well formed that during REM sleep. From SWS we can move into Rapid Eye Movement Sleep, or REM sleep. The name derives from the observation that during this phase of sleep the eyes move together in rapid random jerks as if the person were scanning a picture. Of note is that during REM periods, the nerves from our spinal cord to our arms and legs are actively suppressed so that it’s as if we are paralyzed. (The nerves to the muscles controlling breathing and swallowing are not affected). Watch a dog sleep and you can see REM periods – you see the eyes move under the lids and, if you watch closely, you may even see the paws twitch. It’s appealing to think the dog is dreaming of chasing the neighborhood cat. It is during the REM periods that we have our most vivid dreams. As we sleep through the night, the periods of REM become more frequent at the expense of SWS. We periodically awaken during the night but normally return to SWS.
Various sleep disorders result when the normal sleep mechanisms malfunction. Narcolepsy is a chronic problem that causes excessive periods of daytime sleep in people who otherwise should be well rested. Cataplexy, on the other hand, is a disorder in which emotion triggers severe weakness of the limbs for a period of seconds - as if the centers that suppress muscles during REM sleep are inappropriately activated. Sleep walking may result from not suppressing leg muscles during REM sleep.
It’s not known for certain why we sleep. But it is known for certain that sleep is crucial to our well-being and may play a role in laying down long-term memory. On the other hand sleep deprivation is deleterious to health. Mild deprivation results fatigue-related problems such as work place errors. As deprivation increases so does the severity of side effect and in its most severe form can result in psychosis and seizures. Not everyone requires the same amount of sleep per day, but most of us need at least 6 hours.