f you attached 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 some people, arms, legs and facial muscles will twitch during REM sleep. Periods of sleep other than REM sleep are known 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.

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 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.

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

* missing one night of sleep is not fatal. A person will generally be irritable during the next day and will either 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.

A person who gets just a few hours of sleep per night can experience many of the same problems over time.

Both growth hormones in children 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.

* 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 when we sleep we save considerable amounts of energy than if we were awake.

According to ScienceNewsOnline: Napless cats awaken interest in adenosine, 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

Smoking
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Cigarettes contain more than 4,000 chemicals, including at least 40 that cause cancer. They also contain nicotine; although not one of the cancer-causing agents itself, it is the substance that gets you hooked on cigarettes. In fact, the nicotine in cigarettes is as addictive as heroin or cocaine. Cigarettes -