Many of us are not even aware that we are sleep deprived. But sleep deprivation is a serious problem - getting insufficient sleep can wreak havoc on your health and general well-being. Read on to improve your "sleep literacy" and help you to get the sleep you need.

Sleep helps you to restore and rejuvenate many body functions:

• Memory and learning � Sleep seems to organize memories, as well as help you to recover memories. After you learn something new, sleep may solidify the learning in your brain.
• Mood enhancement and social behaviors - The parts of the brain that control emotions, decision-making, and social interactions slow down dramatically during sleep, allowing optimal performance when awake. REM sleep seems especially important for a good mood during the day. Tired people are often cranky and easily frustrated.
• Nervous system � Some sleep experts suggest that neurons used during the day repair themselves during sleep. When we experience sleep deprivation, neurons are unable to perform effectively, and the nervous system is impaired.
• Immune system � Without adequate sleep, the immune system becomes weak, and the body becomes more vulnerable to infection and disease.
• Growth and development � Growth hormones are released during sleep, and sleep is vital to proper physical and mental development.

How we fall asleep

Our bodies provoke us to sleep by sending messages from our circadian clocks � our inner time-keeping, temperature-fluctuating, enzyme-controlling devices. The circadian clock works in tandem with adenosine, a neurotransmitter that "turns down" many of the bodily processes that make us feel awake. Adenosine is created as our cells create the power needed to move us though the day. As the level of adenosine builds up in our brains, we feel sleepier. Along with corresponding messages from the circadian clock, this alerts us that its time to sleep.

Prior to starting the sleep cycle, we first spend a few moments in a state of "relaxed wakefulness". Most people stay in this stage for less than ten minutes, but the time you spend in relaxed wakefulness can offer clues to how well you are sleeping. If you are falling asleep the moment your head hits the pillow, you are either a very good sleeper, or you are extremely sleep deprived.

Stages of sleep: the sleep cycle

Sleep is a periodic state of rest during which consciousness of the world is interrupted. Additionally, sleep is marked by:

• decreased movement of the skeletal muscles;
• a relaxed posture, usually lying down;
• reduced response to stimulation, such as sounds and touch;
• slowed-down metabolism; and
• complex and active brain wave patterns.

Sleep is divided into two types: REM (Rapid Eye Movement) sleep and NREM (non-REM) sleep. REM sleep is when we dream. NREM sleep is further divided into four stages. A typical night of sleep follows this pattern:

• Stage 1 (Drowsiness) - When you first fall asleep, you are in Stage 1 sleep (Drowsiness). Stage 1 lasts just five or ten minutes. Eyes move slowly under the eyelids, and muscle activity slows down. You are easily awakened during Stage 1 sleep.
• Stage 2 (Light Sleep) -Next, you go into Stage 2 sleep (Light Sleep). In Stage 2, eye movements stop, heart rate slows, and body temperature decreases.
• Stages 3 & 4 (Deep Sleep) - Then you enter Stages 3 and 4 (Deep Sleep). During stages 3 and 4, you are difficult to awaken. People who are awakened during Deep Sleep do not adjust immediately and often feel groggy and disoriented for several minutes after they wake up. Children may experience bedwetting, night terrors, or sleepwalking during Deep Sleep.
• REM sleep (Dream Sleep) - At about 70 to 90 minutes into your sleep cycle, you enter REM sleep. You usually have three to five REM episodes per night. Your eyes jerk rapidly in various directions under your eyelids, thus the name Rapid Eye Movement (REM) Sleep.

The first sleep cycles each night contain relatively short REM periods and long periods of deep sleep. As the night progresses, REM sleep periods increase in length while deep sleep decreases. By morning, people spend nearly all their sleep time in stages 1, 2, and REM.

What happens during the REM sleep stage?

During REM sleep, you dream actively, but your limb muscles are immobile. Your breathing is rapid, irregular, and shallow. Your heart rate increases, your blood pressure rises, males may have penile erections, and females may have clitoral enlargement. Your brain is at least as active during REM sleep as it is when you are awake.

Because your major muscles do not move during REM sleep, you will not act out your dreams. (Sleepwalking occurs during NREM sleep.)

Infants spend about 50 per cent of their sleep time in REM sleep; after infancy, you spend fifteen to twenty per cent of your sleep time in REM sleep.

The importance of deep sleep and dream sleep

Each stage of sleep offers benefits to the sleeper. However, deep sleep is perhaps the most vital stage, as it is the first stage that the brain attempts to recover when we are sleep deprived. If we do not get adequate deep sleep, we experience the strongest effects of sleep deprivation.

Deep sleep allows the brain to go on a little vacation needed to restore the energy we expend during our waking hours. Blood flow decreases to the brain in this stage, and redirects itself towards the muscles, restoring physical energy. Research also shows that immune functions increase during deep sleep.

REM sleep, or dream sleep, is also very important. This stage is associated with processing emotions, retaining memories, and relieving stress. Our brains suspend logic, and we lose all self-awareness � which is why we can experience ridiculous, irrational events in our dreams and believe them to be true.

If our REM sleep is disrupted one night, our bodies don't follow the normal sleep cycle progression the next time we doze off. Instead, we often go through extended periods of REM until we "catch up" on this stage of sleep. But, if deep sleep has been lost as well, our brain attempts to catch up on this stage first � in fact, the brain will try and make up all of the deep sleep it has lost and only half of the REM sleep.

Sleep needs

The recommendation that we need eight hours of sleep to function effectively during the day is not true for all.  Many adults can't sleep eight hours a night, and attempts to do so leads to increased wakefulness in bed.

The amount of sleep that you need depends on a number of factors, including your genetic make-up; the amount of exercise you get; your daily activities; your age; and the quality of your sleep.

Following are some guidelines on how much sleep you or your loved ones might need:

Sleep deprivation

Sleep deprivation is a tricky thing � the more deprived of sleep we are, the more tired, frustrated and hazy we feel � making us less able to recognize the effects of the deprivation and make the necessary changes to improve it.
Some of the signs of sleep deprivation include difficulty waking up in the morning, lack of concentration, falling asleep during work or class, and feelings of moodiness, irritability, depression, or anxiety. Experts say that if you are falling asleep as soon as your head hits the pillow; need an alarm clock to wake up; or feel the need for frequent naps during the day, it is very likely you are sleep deprived.

The effects of sleep deprivation can be felt both physically and mentally. These may include:

• Poor decision-making, poor judgment, increased risk-taking
• Poor performance in school, on the job, and in sports
• Impaired driving performance and more car accidents
• Increased incidence of obesity, diabetes, illness in general, high blood pressure, and heart disease
• Impaired memory, concentration, and ability to learn
• Physical impairment, poor coordination, delayed reaction time
• Anxiety, depression, and other emotional problems
• Magnification of the effects of alcohol on the body
• Exacerbation of the symptoms of ADHD, such as impulse control, irritability, and lack of concentration

Sleep debt

Even if you are getting some sleep every night, you may not be getting as many hours as you need. Being deprived of even just a few hours of sleep each night can create a "sleep debt", a more serious stage of sleep deprivation. A sleep debt can range from one night's very poor sleep to the accumulation of many days of not enough sleep.

A chronic sleep debt can have serious long-term effects, including immune system problems, metabolic changes that can lead to obesity, and hyperactivity. Many of us view sleep as an indulgence or luxury, but in fact, it is a vital key to good health, as important as exercise and eating right.

Luckily, it is possible to get your sleep debt "in the black" by making up for lost sleep and improving future sleep habits.

• To make up a short-term debt, give yourself a few extra hours of sleep every night until you've made up for the sleep you've lost. For example, if you lost 5 hours, add an extra hour every night for 5 straight days.
• To make up a long-term debt, you may need to do a little more work. Experts suggest taking a vacation where you can afford to sleep as much as you want. Do not set any alarms, and allow yourself to sleep as much as you need. You may find yourself sleeping an extreme amount at the beginning of this vacation, but by the end your body should adjust back to the normal number of hours it needs.

Related links for getting the sleep you need

General information on sleep

Brain Basics: Understanding Sleep � A comprehensive overview of the sleep process and its effects on the body. Includes sections on how much sleep we need, the benefits of sleep, dreaming and REM sleep, sleep and circadian rhythms, sleep and disease, and sleep disorders. (National Institute of Neurological Disorders and Stroke)

What is Sleep? � Specifies the behavioral and physiological markers of sleep, discusses the stages of sleep and recent research. (University of Wisconsin Department of Medicine)

Sleep needs and the effects of sleep deprivation

Sheep Dash! � A test of your reaction time. If you're sleep deprived, you will perform more poorly than usual. (BBC, UK)

Epworth Sleepiness Scale Try this online test of sleep debt and get an instant tally of your level of sleepiness. (St Mary Medical Center )

Sleep and children and teens

How Much Sleep Is Enough for My Child? � Tips on how to ensure your child is getting adequate sleep; information on the sleep needs of different age groups. (KidsHealth.org)

Children and Sleep � A comprehensive index of articles that provide information on sleep needs for children of all ages. (New York Online Access to Health)

Delving deeper into sleep and sleep problems

Information About Sleep � A comprehensive teacher's guide that describes the biology of sleep, common sleep misconceptions, sleep requirements (for humans and different types of animals), and sleep disorders. (National Institutes of Health; Sleep, Sleep Disorders, and Biological Rhythms)

Deep into Sleep �discussion of research on sleep deprivation and its effects, which include immune-system deficits, metabolic changes that can lead to obesity, and hyperactivity. Our culture is in a state of "sleep bulimia": we purge on sleep during the week and binge on weekends. (Craig Lambert, Harvard Magazine, July-August 2005)

Tina de Benedictis, Ph.D, Heather Larson, Gina Kemp, M.A., Suzanne Barston, and Robert Segal, M.A., contributed to this article. Last modified on: 12/14/07.