It is actively generated by the discharge of specific neurons in certain parts of the brain.
Sleep affects and in turn is affected by almost every physiological and psychological process.
It is necessary for our mental and physical health. NREM sleep is followed by REM sleep, which lasts about 10 minutes. This 90 minute cycle is repeated 3-6 times each night. In successive cycles the amount of NREM sleep decreases and the amount of REM sleep increases. Each state is comprised of a constellation of biological patterns of activity.