Sleep Tight

Quiet (non-REM) sleep

• In making the transition from wakefulness into light sleep, you spend about five minutes in stage N1 sleep. On the EEG, the predominant brain waves slow to four to seven cycles per second, a pattern called theta waves (see Figure 1). Body temperature begins to drop, muscles relax, and eyes often move slowly from side to side. People in stage N1 sleep lose awareness of their surroundings, but they are easily jarred awake. However, not everyone experiences stage N1 sleep in the same way: if awakened, one person might recall being drowsy, while another might describe having been asleep.
• This first stage of true sleep lasts 10 to 25 minutes. Your eyes are still, and your heart rate and breathing are slower than when awake. Your brain’s electrical activity is irregular. Large, slow waves intermingle with brief bursts of activity called sleep spindles, when brain waves speed up for roughly half a second or longer. Scientists believe that when spindles occur, the brain disconnects from outside sensory input and begins the process of memory consolidation (which involves organizing memories for long-term storage). The EEG tracings also show a pattern called a K-complex, which scientists think represents a sort of built-in vigilance system that keeps you poised to awaken if necessary. K-complexes can also be provoked by certain sounds or other external or internal stimuli. Whisper someone’s name during stage N2 sleep, and a K-complex will appear on the EEG. You spend about half the night in stage N2 sleep.
• Eventually, large, slow brain waves called delta waves become a major feature on the EEG, and you enter deep sleep. Breathing becomes more regular. Blood pressure falls, and the pulse slows to about 20% to 30% below the waking rate. The brain is less responsive to external stimuli, making it difficult to wake the sleeper.

Dreaming (REM) sleep

Dreaming occurs during REM (rapid eye movement) sleep, which has been described as an “active brain in a paralyzed body.” Your brain races, thinking and dreaming, as your eyes dart back and forth rapidly behind closed lids. Your body temperature rises. Your blood pressure increases, and your heart rate and breathing speed up to daytime levels. The sympathetic nervous system, which creates the fight-or-flight response, is twice as active as when you’re awake. Despite all this activity, your body hardly moves, except for intermittent twitches; muscles not needed for breathing or eye movement are quiet.

The role of REM sleep

Just as deep sleep restores your body, scientists believe that REM or dreaming sleep restores your mind, perhaps in part by helping clear out irrelevant information.

Studies of students’ ability to solve a complex puzzle involving abstract shapes suggest the brain processes information overnight; students who got a good night’s sleep after seeing the puzzle fared much better than those asked to solve the puzzle immediately.

Earlier studies found that REM sleep facilitates learning and memory. People tested to measure how well they had learned a new task improved their scores after a night’s sleep. If they were subjected to periodic awakenings that prevented them from having REM sleep, the improvements were lost. By contrast, if they were awakened an equal number of times from deep sleep, the improvements in the scores were unaffected. These findings may help explain why students who stay up all night cramming for an examination generally retain less information than classmates who get some sleep.

About 3-5 times a night, or about every 90 minutes, you enter REM sleep

The first such episode usually lasts for only a few minutes, but REM time increases progressively over the course of the night. The final period of REM sleep may last a half-hour. If you’re deprived of REM sleep and then allowed a subsequent night of undisturbed sleep, you will enter this stage earlier and spend a higher proportion of sleep time in it—a phenomenon called REM rebound.