What is Sleep...
and why do we do it?

We spend about 8 hours/day, 56 hours/week, 240 hours/month and 2,920 hours/year doing it...that's right...SLEEPING. We apparently spend one third of our lives doing nothing. But is sleep really doing nothing? It looks like it...our eyes are closed, our muscles are relaxed, our breathing is regular, and we do not respond to sound or light. If you take a look at what is happening inside of your brain, however, you will find quite a different situation - the brain is very active. You are doing something!

Scientists can record brain activity by attaching electrodes to the scalp and then connecting these electrodes to a machine called an electroencephalograph. The encephalogram (or EEG) is the record of brain activity recorded with this machine. The wavy lines of the EEG are what most people know as "brain waves."

Stages of Sleep

Sleep follows a regular cycle each night. The EEG pattern changes in a predictable way several times during a single period of sleep. There are two basic forms of sleep: rapid eye movement (REM) sleep and non-rapid REM (NREM) sleep. (REM sleep is sometimes called "paradoxical sleep.") Infants spend about 50% of their sleep time in NREM and 50% in REM sleep. Adults spend about 20% of their sleep time in REM and 80% in NREM sleep. Elderly people spend less than 15% of their sleep time in REM sleep.

These lines represent the EEG (electroencephalogram) which shows a record of brain activity; the EMG (electromyogram) shows muscle activity; the EOG (electroculogram) shows eye movements. Look at the differences in the EEG, EMG and EOG during waking, REM sleep (Rapid Eye Movement Sleep) and NREM sleep.

REM Sleep

Most dreaming occurs during REM sleep. During REM sleep, a person's eyes move back and forth rapidly. Sleep researchers discovered this when they woke people up during REM sleep. Often when people in REM sleep wake up, they say that they were just dreaming. The EEG pattern during REM sleep is similar to the EEG pattern when people are awake. However, the muscle activity is very quiet during REM sleep. Muscles are inactive to prevent us from acting out our dreams. This also means that sleepwalkers are not in REM sleep and are not acting out their dreams.

NREM sleep is actually 4 different stages of sleep (Stage 1, Stage 2, Stage 3 and Stage 4) with different EEG patterns. Stages 3 and 4 are sometimes called slow wave sleep.

StageEEG Rate
(Frequency)
EEG Size
(Amplitude)
Awake8-25 HzLow
16-8 HzLow
24-7 Hz
Occasional "sleep spindles"
Occasional "K" complexes
Medium
31-3 HzHigh
4Less than 2 HzHigh
REMMore than 10 HzLow

coasterWhile we are asleep, our brains are on a bit of a "roller-coaster" through different stages of sleep. As we drift off to sleep, we first enter stage 1 sleep. After a few minutes, the EEG changes to stage 2 sleep, then stage 3 sleep, then stage 4 sleep. Then it's back up again: stage 3, stage 2, then a period of REM sleep...then it's back down again, and back up again, and down again...you get the picture. As shown in the figure below, in an 8 hour period of sleep, the brain cycles through these stages about 4-5 times.

Age-related changes in total amount of sleep and REM Sleep
Data from Roffwarg et al., Ontogenetic development of the human sleep-dream cycle, Science, 152:604-619, 1966
Sleep patterns change as people age. As shown in the two graphs above, infants spend more time sleeping and spend a greater percentage of sleep in REM sleep compared with the times of older children and adults. For example, newborn babies sleep about 16 hours per day and spend about 50% of that time in REM sleep. Older people (50-85 years old) sleep only 5.75-6 hours per day and spend 13.8-15% of that time in REM sleep.
As you might expect, as children grow, they spend less time sleeping during the day. The graph below illustrates how nighttime and daytime sleep time changes with age.

Data from Howard, B.J. and Wong, J. Sleep disorders, Pediatrics in Review, 22:327-341, 2001.

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Did you know?

Did you ever think about how much you sleep and dream? The "average" human sleeps about 8 hours every day. That's one third of your life! In other words, you sleep for about 122 days every year. A 75 year old person would have spent a total of about 25 years asleep. There is a wide range in the amount of time different animals spend sleeping.

As for dreaming...we enter REM sleep about 5 times in an average 8 hour period of sleep. If we assume that we dream during each of these REM periods, then in one year, we will have had 1,825 dreams! Of course we don't remember all of these dreams. A 75 year old person would have about 136,875 dreams!

Why Sleep?

Why sleep at all? It seems like a big waste of time. Think of all you could be doing if you did not sleep. Nevertheless, sleep appears to be necessary. There is a continuing debate about why we sleep. Why do most animals sleep? How much sleep is required?

Most "higher" animals appear to sleep during some portion of the day and/or night. - they are quiet; they rest; they do not move. Scientists have recorded sleep-like EEG patterns in birds, reptiles and mammals, but it is not clear if insects and other invertebrates also sleep.

No one knows for sure why we sleep, but here are 2 basic theories:

  1. Sleep has a restorative function.
  2. Sleep has an adaptive function.

Sleep as a Restorative Process

This theory of sleep suggests that sleep helps the body recover from all the work it did while an animal was awake. Experiments have shown that the more physical exercise an animal does, the more NREM an animal will have. Also, if people are deprived of NREM by waking them up each time they get to stage 4 sleep, then they complain of being physically tired. If people are deprived of REM sleep by waking them up each time the have REM type EEG patterns, they can get anxious and irritable. If animals are deprived of REM for several days and then allowed to get an undisturbed period of sleep, animals will go into "REM rebound" - this is when REM periods of sleep will happen more often and for a longer time than normal.

Sleep, especially REM sleep, has also been thought to be important for memory and learning. It is possible that sleep helps form memories.

Sleep as an Adaptive Process

Sleep may have developed because of a need of animals to protect themselves. For example, some animals search for food and water during the day because it is easier to see when the sun is out. When it is dark, it is best for these animals to save energy, avoid getting eaten, and avoid falling off a cliff that they cannot see. It is interesting to note which animals sleep the most and which sleep the least. In general, animals that serve as food for other animals sleep the least.

Highlights from the National Sleep Foundation's 2001 Sleep in America telephone survey of 1,004 adults:

  • 63% of the surveyed adults get less than the recommended eight hours of sleep per night; 31% get less than seven hours.
  • 40% of surveyed adults in the US report having trouble staying awake during the day.
  • Over the last five years, people in the US have worked more and slept less.
  • Eight out of ten people said that they would sleep more if they knew it would improve their health and memory.

Did you know?

  • Sleep disorders affect up to 70 million people in the United States. This costs about $100 billion each year in accidents, medical bills and lost work. (Statistic from Brain Facts, Society for Neuroscience, 2002)
  • Sleepwalking is also known as "somnambulism"; sleeptalking is also known as "somniloquy."

Try these sleep experiments on your own.
Do you like interactive word search puzzles? Make sure your browser is "java-enabled" and try this one:

  • Sleep Puzzle
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    For more information about sleep, see:
    1. National Center on Sleep Disorders Research
    2. The Science of Sleep - CBS News
    3. TalkAboutSleep.com
    4. Brain Basics - Understanding Sleep
    5. National Sleep Foundation
    6. Tossing and Turning No More: How to Get a Good Night's Sleep
    7. Insomniacs.co.uk

    They said it!

    "Early to bed, and early to rise,
    makes a man healthy, wealthy, and wise."
    --- Benjamin Franklin, 1758 (in Poor Richard's Almanack)

    "The woods are lovely, dark and deep.
    But I have promises to keep,
    and miles to go before I sleep."
    --- Robert Frost, 1923 (in Stopping By Woods On A Snowy Evening)

    "Sleep is better than medicine."
    --- English Proverb

    "The beginning of health is sleep."
    --- Irish Proverb

    "In sleep we are all equal."
    --- Spanish Proverb

    "Disease and sleep keep far apart."
    --- Welsh Proverb

    "Dreaming of eating will not satisfy the hungry."
    --- African Proverb

    "Sleep is an acquired habit. Cells don't sleep. Fish swim in the water all night. Even a horse doesn't sleep. A man doesn't need any sleep."
    --- Thomas Edison, inventor

    "I never use an alarm clock. I can hardly wait until five a.m. In the army I always woke before reveille. I hate sleeping. It wastes time."
    --- Isaac Asimov, science fiction writer

    BACK TO: Exploring the Nervous System Table of Contents

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