The need for sleep is universal, but why? Its purposes and mechanisms are not fully understood. Still, scientists have learned a lot about the physiology and biochemistry of sleep in recent decades—for instance, how it restores the nervous, muscular, skeletal, hormonal and immune systems and allows us to consolidate memories.
Sleep is an activity that has a structure. It is not a simple process of falling asleep, snoozing soundly and awakening. Nobody sleeps “like a log.” It is not abnormal to wake briefly, even a few times a night, and not remember it. You move in your sleep—muscles, eyes—and your heart rate and blood pressure vary.
Sleep consists of four stages, which you cycle through usually four or five times a night:
Stage 1: Very light sleep. Eye movements slow and muscle activity decreases. It normally makes up only five percent of your nightly sleep.
Stage 2: Electrical activity in the brain slows, with occasional bursts of rapid brain waves. This accounts for about half of total sleep time.
Stage 3: This deep, restorative sleep features very slow brain waves, called delta waves. Breathing slows and blood pressure and body temperature drop. If something wakes you, you’ll be confused and groggy. After this stage, you normally cycle back through stage 2 and enter REM sleep. This stage makes up about 25 percent of sleep and decreases with age.
REM (rapid eye movement) sleep. This is when you dream. Darting eye movements begin, heart rate and blood pressure rise, and limbs are immobilized. REM periods lengthen as the night progresses and account for about 20 percent of total sleep time.
What is “enough” sleep? Enough is whatever makes you feel refreshed and alert the next day. Most adults do best with six to eight hours—but for some it’s nine or ten, for others six or even less. How well you sleep is as important as how long. Though studies have linked the six-to-eight hour range to lower death rates, what that means for individuals is unclear. What’s most important is to get whatever is enough sleep for you.
Published October 01, 2013