Two Main Types of Sleep

Sleep is the natural state of lowered consciousness and reduced metabolism. Sleep consumes about one third of an average person's life.

Since the early 20th century, human sleep has been described as a succession of five recurring stages: four non-REM stages and the REM stage. A sixth stage, waking, is often included. In other words, Rapid Eye Movement (REM) Sleep and Non-Rapid Eye Movement (non-REM) Sleep are the two main types of sleep.

People normally cycle through the four stages of non-REM sleep, usually followed by a brief interval of REM sleep, 5 or 6 times every night. Sleep progresses from stage 1 (the lightest level, during which the sleeper can be awakened easily) to stage 4 (the deepest level, during which the sleeper can be awakened only with difficulty). In stage 4, blood pressure is at its lowest, and heart and breathing rates are at their slowest.

During REM sleep, electrical activity in the brain is unusually high, somewhat resembling that during wakefulness. The eyes move rapidly, and muscles may jerk involuntarily. The rate and depth of breathing increase, but the muscles, except for the diaphragm, are greatly relaxed—more so than during the deepest levels of non-REM sleep. Most dreaming occurs during REM sleep. Most talking during sleep, night terrors, and sleepwalking occur during stages 3 and 4.

People who feel exhausted during the day may suffer from a sleep disorder called obstructive sleep apnea. This form of interrupted sleep affects about 4 percent of middle-aged women and 2 percent of middle-aged men. It can have harmful consequences.