brain, spinal cord, and nerve disorders
Overview of Sleep
The sleep cycle
All sleep is not the same. There are two main types of sleep: Rapid eye movement (REM) sleep Nonrapid eye movement (NREM) sleep, which has three stages People normally cycle through the three stages of NREM sleep (stages N1 through N3), usually followed by a brief interval of REM sleep, every 90 to 120 minutes or several times every night. Throughout the night, people wake up briefly (called stage W) but are typically unaware of being awake. NREM sleep:NREM sleep accounts for about 75 to 80% of total sleep time in adults. Sleep progresses from stage 1 (the lightest level, when the sleeper can be awakened easily) to stage 3 (the deepest level, when the sleeper can be awakened with greater difficulty). In stage 3, blood pressure is at its lowest, and heart and breathing rates are at their slowest. People perceive stage 3 as high-quality sleep. REM sleep:Electrical activity in the brain is unusually high, somewhat resembling that during wakefulness. The eyes move rapidly, and certain muscles are paralyzed so that voluntary movement is impossible. However, some muscles may twitch involuntarily. The rate and depth of breathing increase. The most vivid dreaming occurs during REM sleep. Most talking during sleep, night terrors, and sleepwalking occur during stage 3 NREM sleep. People normally cycle through distinct stages of sleep every 90 to 120 minutes during the night: three stages of nonrapid eye movement sleep (N) and one stage of rapid eye movement (REM) sleep. Relatively little time is spent in stage N1 (shallow) sleep. The greatest time is spent in stage N2 sleep. Deep sleep (stage N3) occurs mostly during the first half of the night. More time is spent in REM sleep as the night progresses. Brief awakenings (called stage W) occur throughout the night, but the sleeper is typically unaware of most of them. 