For centuries, scientists scrutinized minute aspects of human activity, but showed little interest in the time that people spent in sleep. Sleep seemed inaccessible to medical probing, a subject best suited to poets and dream interpreters who could conjure meaning out of the void. It was thought to be no more than a natural reaction to the darkness and silence of night, when the brain could no longer react to the sensory stimulation of the day.

Scientists believed the brain was simply “turned off” and remained unchanged throughout the night. All that changed in the 1930s, when researchers found they could place sensitive electrodes on the scalp and record the signals produced by electrical activity in the brain to create electroencephalograms, or EEGs. When this technique revealed lively brain activity even in sleeping subjects, scientists realized that sleep did not represent a void, but was in fact a dynamic time, incorporating different stages with markedly different patterns of brain waves.

Further study revealed that it is a period when the brain consolidates memories and cleans out toxins, when growth and development occur, and when the immune system surveys the body for potential problems. Far from being a luxury, sleep is now known to be essential, required for health and optimal functioning. Sleep deprivation can be used as a form of torture. Yet many of us continue to get inadequate sleep, either because we deliberately deprive ourselves of sleep or because we seem unable to get a good night’s rest.

But what happens during sleep? There are different stages of sleep, what scientists call “sleep cycle,” meaning the patterns of alternating sleep stages you pass through during the night. Scientists divide sleep into two major types: REM (rapid eye movement) sleep or dreaming sleep (occupies 75–80% of total sleep each night), and non-REM (non-rapid eye movement) or quiet sleep (occupies 20–25% of total sleep each night). A sleep cycle consists in 4 stages: 1 & 2 -light sleep-, 3 -deep sleep- (all three stages are non-REM sleep) and 4 (REM sleep).

A sleep cycle lasts about 90-120 minutes and during that time we move through all 4 stages of sleep.

Stage N1 (light sleep). You spend about 5 minutes in stage N1 sleep. It is the transition between waking and sleep. Body temperature begins to drop, muscles relax, and eyes often move slowly from side to side. If awakened, a person will claim was never asleep. On the EEG, the predominant brain waves show a pattern called theta waves.

Stage N2 (light sleep). It 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. You spend about half the night in stage N2 sleep. 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.

Stage N3 (deep sleep, or slow-wave sleep). It lasts for about 30 minutes. Breathing becomes more regular. Blood pressure falls and the pulse slows. The brain is less responsive to external stimuli, making it difficult to wake the sleeper. The major feature of this stage on the EEG are large, slow brain waves called delta waves. Deep sleep reduces your sleep drive, and provides the most restorative sleep of all the sleep stages.

This is why if you take a short nap during the day, you’re still able to fall asleep at night. But if you take a nap long enough to fall into deep sleep, you have more difficulty falling asleep at night because you reduced your need for sleep. At the beginning of this stage, the pituitary gland releases a pulse of growth hormone that stimulates tissues rebuilding and repair.

Researchers have also detected increased blood levels of substances that activate your immune system, raising the possibility that deep sleep helps the body defend itself against infection. Deep sleep seems to be a time for your body to renew and repair itself. When you sleep after a period of sleep deprivation, you pass quickly through the lighter sleep stages into the deeper stages and spend a greater proportion of sleep time there.

This suggests that deep sleep plays a large part in restoring alertness and fills an essential role in a person’s optimal functioning. Normally, young people spend about 20% of their sleep time in stretches of deep sleep lasting up to half an hour, but deep sleep is nearly absent in most people over age 65.

Dreaming (REM) sleep. It typically begins about 90 minutes after you first fall asleep, with the first REM cycle lasting about 10 minutes. Each successive REM cycle last longer, with the final REM stage lasting up to 1 hour. Most people experience 3 to 5 REM sleep each night.

Dreaming occurs during REM 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. 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. REM sleep plays an important role in learning and memory function, since this is when your brain consolidates and processes information from the day before so that it can be stored in your long-term memory.

If your body doesn’t get a chance to properly recharge – by cycling through REM and NREM – you’re already starting the next day at a disadvantage. You might find yourself feeling drowsy, irritable or sometimes depressed; struggling to take in new information at work, remembering things or making decisions; craving more unhealthy foods, which could cause weight gain. If this happens night after night, it places a tremendous strain on your nervous system, body and overall health. Commit to getting at least 7 hours of good quality sleep every night, to keep your mind and body in balance.

As Thomas Dekker used to say:


I wish you all the best,
Dr. Valeria Acampora