Sleep is one of those funny things about being a human being -- you just have to do it. Have you ever wondered why? And what about the crazy dreams, like the one where a bad person is chasing you and you can't run or yell. Does that make any sense?

If you have ever wondered about why people have to sleep or what causes dreams, then read on. In this edition of HowStuffWorks, you'll find out all about sleep and what it does for you.

Characteristics of Sleep
We all know how sleep looks -- when we see someone sleeping, we recognize the following characteristics:

• If possible, the person will lie down to go to sleep.

• The person's eyes are closed.

• The person doesn't hear anything unless it is a loud noise.

• The person breathes in a slow, rhythmic pattern.

• The person's muscles are completely relaxed. If sitting up, the person may fall out of his or her chair as sleep deepens.

• During sleep, the person occasionally rolls over or rearranges his or her body. This happens approximately once or twice an hour. This may be the body's way of making sure that no part of the body or skin has its circulation cut off for too long a period of time.

In other words, a sleeping person is unconscious to most things happening in the environment. The biggest difference between someone who is asleep and someone who has fainted or gone into a coma is the fact that a sleeping person can be aroused if the stimulus is strong enough. If you shake the person, yell loudly or flash a bright light, a sleeping person will wake up.

For any animal living in the wild, it just doesn't seem very smart to design in a mandatory eight-hour period of near-total unconsciousness every day. Yet that is exactly what evolution has done. So there must be a pretty good reason for it!

How Much Sleep Do I Need?Most adult people seem to need seven to nine hours of sleep a night. This is an average, and it is also subjective. You, for example, probably know how much sleep you need in an average night to feel your best. The amount of sleep you need decreases with age. A newborn baby might sleep 20 hours a day. By age four, the average is 12 hours a day. By age 10, the average falls to 10 hours a day. Senior citizens can often get by with six or seven hours a day.

Who Sleeps?
Reptiles, birds and mammals all sleep. That is, they become unconscious to their surroundings for periods of time. Some fish and amphibians reduce their awareness but do not ever become unconscious like the higher vertebrates do. Insects do not appear to sleep, although they may become inactive in daylight or darkness.

By studying brainwaves, it is known that reptiles do not dream. Birds dream a little. Mammals all dream during sleep.

Different animals sleep in different ways. Some animals, like humans, prefer to sleep in one long session. Other animals (dogs, for example) like to sleep in many short bursts. Some sleep at night, while others sleep during the day.

Really?Cows can sleep while standing up, but they only dream if they lie down! Whales and dolphins are "conscious breathers," and they need to keep breathing while they sleep, so only one half of the brain sleeps at a time! (Click here for details.)

Sleep and the Brain
If you attach an electroencephalograph to a person's head, you can record the person's brainwave activity. An awake and relaxed person generates alpha waves, which are consistent oscillations at about 10 cycles per second. An alert person generates beta waves, which are about twice as fast.

During sleep, two slower patterns called theta waves and delta waves take over. Theta waves have oscillations in the range of 3.5 to 7 cycles per second, and delta waves have oscillations of less than 3.5 cycles per second. As a person falls asleep and sleep deepens, the brainwave patterns slow down. The slower the brainwave patterns, the deeper the sleep -- a person deep in delta wave sleep is hardest to wake up.

At several points during the night, something unexpected happens -- rapid eye movement (REM) sleep occurs. Most people experience three to five intervals of REM sleep per night, and brainwaves during this period speed up to awake levels. If you ever watch a person or a dog experiencing REM sleep, you will see their eyes flickering back and forth rapidly. In many dogs and some people, arms, legs and facial muscles will twitch during REM sleep. Periods of sleep other than REM sleep are know as NREM -- non-REM -- sleep.

REM sleep is when you dream. If you wake up a person during REM sleep, the person can vividly recall dreams. If you wake up a person during NREM sleep, generally the person will not be dreaming.

You must have both REM and NREM sleep to get a good night's sleep. A normal person will spend about 25 percent of the night in REM sleep, and the rest in NREM. A REM session -- a dream -- lasts five to 30 minutes.

Medicine can hamper your ability to get a good night's sleep. Many medicines, including most sleeping medicines, change the quality of sleep and the REM component of it.

Why Sleep?
One way to understand why we sleep is to look at what happens when we don't get enough:

• As you know if you have ever pulled an all-nighter, missing one night of sleep is not fatal. A person will generally be irritable during the next day and will either slow down (become tired easily) or will be totally wired because of adrenalin.

• If a person misses two nights of sleep, it gets worse. Concentration is difficult, and attention span falls by the wayside. Mistakes increase.

• After three days, a person will start to hallucinate and clear thinking is impossible. With continued wakefulness a person can lose grasp of reality. Rats forced to stay awake continuously will eventually die, proving that sleep is essential.

Two other things are known to happen during sleep. Growth hormone in children is secreted during sleep, and chemicals important to the immune system are secreted during sleep. You can become more prone to disease if you don't get enough sleep, and a child's growth can be stunted by sleep deprivation.

But the question remains -- why do we need to sleep? No one really knows. There are all kinds of theories, including these:

• Sleep gives the body a chance to repair muscles and other tissues, replace aging or dead cells, etc.

• Sleep gives the brain a chance to organize and archive memories. Dreams are thought by some to be part of this process.

• Sleep lowers our energy consumption, so we need three meals a day rather than four or five. Since we can't do anything in the dark anyway, we might as well "turn off" and save the energy.

• According to this article, sleep may be a way of recharging the brain, using adenosine as a signal that the brain needs to rest: "Since adenosine secretion reflects brain cell activity, rising concentrations of this chemical may be how the organ gauges that it has been burning up its energy reserves and needs to shut down for a while." Adenosine levels in the brain rise during wakefulness and decline during sleep.

Tips to Improve Your Sleep Exercise regularly. Exercise helps tire and relax your body. Don't consume caffeine after 4:00 p.m. or so. Avoid other stimulants like cigarettes as well. Avoid alcohol before bedtime. Alcohol disrupts the brain's normal patterns during sleep. Try to stay in a pattern with a regular bedtime and wakeup time, even on weekends.

Dreams
Why do we have such crazy, kooky dreams? Why do we dream at all for that matter? According to Joel Achenbach in his book Why Things Are:

The brain creates dreams through random electrical activity. Random is the key word here. About every 90 minutes the brain stem sends electrical impulses throughout the brain, in no particular order or fashion. The analytic portion of the brain -- the forebrain -- then desperately tries to make sense of these signals. It is like looking at a Rorschach test, a random splash of ink on paper. The only way of comprehending it is by viewing the dream (or the inkblot) metaphorically, symbolically, since there's no literal message.

This doesn't mean that dreams are meaningless or should be ignored. How our forebrains choose to "analyze" the random and discontinuous images may tell us something about ourselves, just as what we see in an inkblot can be revelatory. And perhaps there is a purpose to the craziness: Our minds may be working on deep-seated problems through these circuitous and less threatening metaphorical dreams.

• Dreams tell a story. They are like a TV show, with scenes, characters and props.

• Dreams are egocentric. They almost always involve you.

• Dreams incorporate things that have happened to you recently. They can also incorporate deep wishes and fears.

• A noise in the environment is often worked in to a dream in some way, giving some credibility to the idea that dreams are simply the brain's response to random impulses.

• You usually cannot control a dream -- in fact, many dreams emphasize your lack of control by making it impossible to run or yell. (However, proponents of lucid dreaming try to help you gain control.)

Dreaming is important. In sleep experiments where a person is woken up every time he/she enters REM sleep, the person becomes increasingly impatient and uncomfortable over time.

For more information, check out the links on the next page.

Lots More Information!

Related HowStuffWorks Articles

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• Do whales and dolphins sleep?
• How Your Brain Works
• How Caffeine Works

More Great Links

• The Sleep Medicine Home Page - lots of links
• Napless cats awaken interest in adenosine
• Experts: Melatonin hormone unproven as sleep aid
• Brain Basics: Understanding Sleep
• Yahoo! Directory: Sleep and Dreams