Know somebody who likes to brag that he can get by on six hours of sleep a night?

Tell him that men who sleep less than seven hours a night have a 26 percent greater death rate over a two-decade period than men who sleep seven to eight hours a night.

And children who don't get enough sleep are more likely to be overweight and have behavioral problems.

And people who do rotating-shift work have lower levels of the hormone serotonin, low levels of which are associated with anxiety and depression.

These findings, all published in the journal Sleep, are part of a rapidly expanding body of knowledge about the physiology of sleep and the importance of adequate sound sleep to good health.

"Shift work was just added to the list of risk factors for cancer by the CDC (Centers for Disease Control and Prevention)," says Dr. Jerrold Kram, a member of the board of directors of the National Sleep Foundation. "It just suggests the increasing recognition of how profoundly sleep affects our lives."

And it's not just arcane statistics about risk factors and sleep that are accumulating. There are 83 recognized sleep disorders, including sleep apneas, insomnias, circadian-rhythm disturbances, narcolepsy, restless leg syndrome and plain old wake-the-neighborhood snoring. Physicians such as Kram are putting this knowledge to use, making sleep medicine one of the fastest- growing medical specialties over the past decade.

The American Academy of Sleep Medicine accredited the first clinical sleep lab in 1977. The idea of community medical centers where patients would be hooked up to monitors while they punched their pillows, snored and dreamed about showing up for college exams naked, grew slowly at first — by 1996, there were just 300 AASM-accredited sleep centers — but the concept has exploded in the past decade, resulting in more than 1,000 accredited centers today and many more unaccredited centers.

YOU NEED ALL FIVE STAGES OF SLEEP

"We used to think that sleep was a dormant period of time, and we're finding out that there are a whole lot of things that go on during sleep," says Dr. David Ostransky, a pulmonology and sleep-medicine specialist in Fort Worth, Texas.

Saying "a whole lot" goes on during sleep is like saying that war is "unpleasant."

There are five stages of sleep, according to the National Sleep Foundation, four non-REM stages and REM (rapid eye movement) sleep, and sleepers cycle through the stages about every 90 to 100 minutes.

Stage 1 is the transitional stage, when you're between waking and sleep. Your brain waves and muscle activity slow. Sometimes, people's bodies jerk just before they fall asleep.

Stage 2 is a light sleep stage. Eye movements cease. Body temperature drops, and heart and brain activity slows. National Sleep Foundation material says there are periods of muscle tone and muscle relaxation, and that occasional brain-wave spikes, called sleep spindles, occur during this stage of sleep.

Stages 3 and 4 are called delta sleep. These are the deep sleep stages, and this is when body restoration and repair occurs. Temperature drops even further during this phase, brain waves are slow and there is decreased muscle tone. Fibromyalgia may be associated with poor delta sleep, Ostransky says. People woken during delta sleep are often groggy and disoriented. Night terrors occur during this sleep stage.

Stage 5 is REM sleep, a period of fast brain waves; rapid, shallow breathing; and the rapid eye movements it's named for. Dreaming, believed to be a way of organizing the day's experiences, Ostransky says, occurs during REM sleep. Have you ever wanted to scream during a nightmare and been unable to? Muscles become temporarily paralyzed during REM.

These stages of sleep are repeated four to six times during the night, but not in exactly the same ratio. The first REM sleep is short, just seven minutes or so, but REM sleep takes up a larger portion of the cycles as the night goes on, Ostransky says, which is why you're often dreaming when your alarm clock goes off.

Besides the amount of sleep, the distribution of sleep stages is important for health. People who don't get adequate delta sleep — or REM sleep — wake up feeling unrestored, Ostransky says.

DON'T MESS WITH YOUR CIRCADIAN RHYTHM

The suprachiasmatic nucleus, a region in the hypothalamus, regulates the body's sleep/wake cycle, or circadian rhythm, and it needs to be kept in adjustment, just like the alarm clock beside your bed needs to be kept adjusted.

Cycles of light and dark are what keep the suprachiasmatic nucleus properly set so that you will go to sleep at 11 and wake up at 7. The human body, deprived of clues such as sunlight and clocks, wouldn't naturally keep a 24-hour schedule. Many people who are totally blind, in fact, have a circadian rhythm disorder, continually advancing their sleep/wake schedule forward.

Teenagers are particularly prone to circadian-rhythm problems, and their night-owl tendencies are at least partly biological, says Kram, the sleep foundation spokesman.

Light is the chief, but not the only, clue for the body's inner clock, according to the American Association of Sleep Medicine Web site. Zeitgebers are the name for other circadian-rhythm influences, and they include meals, exercise and routine activities.

People with chronic insomnia may be helped by sticking to regular times for meals, exercise and bedtime routines.

WITHOUT SLEEP, YOUR HORMONES GET OUT OF WHACK

Have you ever been inside the cockpit of a plane? So many dials.

The human body is a far more complicated piece of machinery than a plane, and it has a complex system of hormones that are constantly being adjusted to keep endocrine, metabolic and other body systems functioning properly.

Many of these regulatory hormones are secreted at night or during periods of sleep, and sleep disorders or life situations such as shift work can affect the proper sequence of hormone release, the sleep foundation says.

Melatonin, a hormone secreted by the pineal gland in the brain in reaction to darkness, helps promote sleep, but has other functions as well. Disruptions in melatonin production may be the reason why shift work is associated with an increased risk of cancer.

Growth hormone, necessary for growth in children, is released during the deep delta sleep stages, Ostransky says. Growth hormone is important for adults, too, who require it to repair the body and regulate muscle mass.

The stress hormone cortisol varies during the night, Kram says. It falls as the body enters sleep and then rises in the morning before waking.

Hormones involved in the reproductive cycle, including luteinizing hormone and follicle-stimulating hormone, are released during sleep, according to the foundation.

And medical researchers believe that the hormones ghrelin and leptin, which help signal hunger and satiety, are affected by sleep. Low levels of leptin in children and adults who get inadequate sleep may be the reason for the link between insufficient sleep and obesity. Kram says people with interrupted sleep are more likely to develop insulin resistance because the balance of these hormones is off.

TIPS FOR GETTING BETTER SLEEP

So you're convinced that you need seven to eight hours of sound sleep a night, but your body just doesn't want to cooperate. Here's how to start solving the problem: