The invention of the electroencephalograph allowed scientists to study sleep in ways that were not previously possible. Further studies of human sleep have demonstrated that sleep progresses through a series of stages in which different brain wave patterns are displayed.
Scientists divide sleep into two major types:
• Non-REM, or quiet, sleep
• Rapid eye movement (REM), or dreaming, sleep
Surprisingly, they are as different from one another as sleeping is from waking.
Sleep specialists have called non-REM sleep “an idling brain in a movable body.” During this phase, thinking and most physiological activities slow down, but movement can still occur, and a person often shifts position while sinking into progressively deeper stages of sleep.
To an extent, the convention of describing people “descending” or “dropping” into sleep actually parallels changes in brain wave patterns at the onset of non-REM sleep. When you are awake, billions of brain cells receive and analyze sensory information, coordinate behavior, and maintain bodily functions by sending electrical impulses to one another, which can be recorded as brain waves. Similar to our example of a handful of stones thrown into a pond, the multitude of different waves generated when we’ll fully awake interfere with each other, cancel each other out, and result in the EEG recording an irregular scribble of electrical activity.
Once your eyes are closed and your nerve cells no longer receive visual input, brain waves settle into a steady and rhythmic pattern of about eight to twelve cycles per second. This is the alpha wave pattern, characteristic of calm, relaxed wakefulness.
Unless something disturbs the process, you will soon proceed smoothly through the four stages of non-REM sleep.
In making the transition from wakefulness into light sleep, you spend about five minutes in Stage 1 sleep. On the EEG, the predominant brain waves slow to four to seven cycles per second, a pattern called theta waves. The body temperature begins to drop, muscles become relaxed, and eyes often move slowly from side to side. People in Stage 1 sleep lose awareness of their surroundings, but they are easily jarred into wakefulness. However, not everyone experiences Stage 1 sleep in the same way; if awakened, one person might recall being only drowsy, while another might describe having been asleep.
This is the first stage of established sleep. The first time it occurs it lasts ten to twenty-five minutes before you progress to another stage of sleep. Your eyes are usually still, and your heart rate and breathing are slower than when you’re awake. Your brain’s electrical activity is irregular. Intermediate-size brain waves intermingle with brief bursts of fast activity called sleep spindles, when brain waves speed up for roughly half a second or longer.
About every two minutes, EEG tracings show a pattern called a K-complex, which scientists think represents a sort of built-in vigilance system that keeps you poised to be awakened if necessary. K-complexes can be provoked by certain sounds or other external or internal stimuli. Whisper someone’s name during Stage 2 sleep, and a K-complex will appear on the EEG. Stage 2 sleep is commonly seen in the transition between other sleep stages; overall, about half the night is usually spent in Stage 2 sleep.
Stages 3 and 4, or Deep Sleep.
As you travel into deeper sleep, fewer and fewer of the brain’s processing centers stay active and the firing of the remaining active brain cells becomes more coordinated. From our pond example, this would be like when your supply of stones starts to run out and you throw fewer at a time and less frequently, finally throwing only one at a time to keep from running out. The resulting waves become bigger and more distinct. Eventually, large slow brain waves called delta waves become the major feature on the EEG.
Together, Stages 3 and 4 are known as deep sleep, or slow-wave sleep. Stage 4 sleep occurs when at least half of the brain waves are delta waves. During deep sleep, your breathing slows and becomes more regular. Your blood pressure and pulse fall to about 20 to 30 percent below their waking rates.
Your brain becomes less responsive to external stimuli, making it difficult to awaken. Deep sleep seems to be a time for your body to renew and repair itself. Less blood flow is directed toward your brain, which cools measurably. At the beginning of this stage, the pituitary gland releases a pulse of growth hormone that stimulates tissue growth and muscle repair. Researchers have also detected increased blood levels of substances such as interleukin that activate your immune system, raising the possibility that deep sleep helps the body defend itself against infection.
Normally, young adults spend about 20 percent of their sleep time in up to half-hour stretches of slow-wave sleep, but slow-wave sleep declines sharply in most people over age sixty-five. When a sleep-deprived person finally gets some sleep, he or she passes quickly through the lighter sleep stages into the deeper stages and spends a greater proportion there, suggesting that slow-wave sleep is the restorative portion of sleep you need to feel refreshed.
Bed-wetting and sleepwalking are most likely to occur at the end of stage 4 sleep.
Most dreaming occurs during the fifth stage of sleep, known as rapid eye movement (REM) sleep. REM sleep is characterized by eye movement, increased respiration rate and increased brain activity. REM sleep is also referred to as paradoxical sleep because while the brain and other body systems become more active, muscles become more relaxed. Dreaming occurs due because of increased brain activity, but voluntary muscles become paralyzed.
Dreaming occurs during REM sleep, which has been described as an “active brain in a paralyzed body.” Your brain races as your eyes dart back and forth rapidly behind closed lids. Your body temperature rises. Unless you have a circle or other physical problems, the penis or clitoris become erect. 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, because muscles not needed for breathing or eye movement are temporarily paralyzed.
We don’t know whether dreams have deep meaning. But we do know that just as deep sleep restores your body, dreaming sleep restores your mind, perhaps in part by helping clear out irrelevant information.
Studies show, for example, that REM sleep facilitates learning and memory. People tested to measure how well they have learned a new task to improve their scores after a night’s sleep. If repeatedly roused from REM sleep, however, the improvements are lost. On the other hand, if they are awakened an equal number of times from slow-wave sleep, the improvements in the scores are unaffected. Such 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.
Three to five times a night, or about every ninety minutes, a sleeper enters 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 half an hour. Altogether, REM sleep makes up about 25 percent of total sleep in young adults. If someone who has been deprived of REM sleep is left undisturbed for a night, he or she enters this stage earlier and spends a higher proportion of sleep time in it – a phenomenon called REM rebound.
The Sequence of Sleep Stages
It is important to realize, however, that sleep does not progress through these stages in sequence. Sleep begins in stage 1 and progresses into stages 2, 3 and 4. After stage 4 sleep, stage 3 and then stage 2 sleep are repeated before entering REM sleep. Once REM sleep is over, the body usually returns to stage 2 sleep. Sleep cycles through these stages approximately four or five times throughout the night.
On average, we enter the REM stage approximately 90 minutes after falling asleep. The first cycle of REM sleep might last only a short amount of time, but each cycle becomes longer. REM sleep can last up to an hour as sleep progresses.
Cumulative Sleep Debt
Your body needs a certain amount of sleep to function at its best. For simplicity’s sake, let’s say it’s eight hours. When you fail to get eight hours of sleep, you start to accumulate a sleep debt—somewhat similar to what occurs in your bank account when you regularly withdraw more money than you deposit. If you sleep five hours on Monday, six hours on Tuesday, and seven on Wednesday, then you’ve built up a sleep debt of six hours (3 2 1 6). The greater your sleep debt over a few days, the stronger the drive for sleep. Your increasing sleepiness is your body’s natural attempt to force you to get your eight hours of sleep.
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- Each phase may have a distinct physiological function (findbatteries.wordpress.com)
- Sleeping (spaceandscience.wordpress.com)
- What your Dreams Say: Can New App Decode your REM Cycles? (scienceworldreport.com)
- Sleep Cycle Apps: Precise, or Placebo? (lions-talk-science.org)
- How Dreaming Changed Human Evolution (psychologytoday.com)