Most of the progress in understanding sleep mechanics has been made in the last 50 years. What has been learned explodes some long-standing misconceptions. One is the assumption that since many bodily functions slows down during rest; sleep is little more than a state of inactivity.

       By studying brain-wave patterns, medical researchers have learned that there are repeated cycles and stages of sleep. Far from being inactive, the human brain runs at high speed during certain periods of sleep. Healthful sleep involves going through these cycles four or more times every night and spending a sufficient amount of time in each cycle.

         A normal night’s sleep is most easily divided into 2 types: what is commonly called REM [rapid eye movement, or dream] sleep and non-REM [nondream] sleep. You can tell a person is in REM sleep when the bulge of his eyeballs can be seen rapidly moving under his eyelids.

        Non-REM sleep can be further divided into 4 stages. After lying down, you gently enter stage one –drowsiness or shallow sleep. During this stage your muscles relax and your brain waves are irregular and rapid. Its first occurrence each night typically lasts between 30 minutes and 7 minutes. When you move into stage 2 –true sleep –where you will spend 20 percent of the night, but you are unaware of your surroundings and cannot see even if your eyes are open.

         Next come stages three and four –deeper to deepest sleep. Here, in what is also called delta sleep, your brain produces large, slow waves. It is now that your body is most difficult to rouse, as most of your blood is directed to the muscles. During this time [usually about 50 percent of the night], body recovery and repair take place, and it is during delta sleep that young bodies grow. It is important to note that anyone, young or adult, who does not experience the deeper delta stages will likely feel fatigued, apathetic, or even depressed the next day.

        Finally, each cycle is completed by the radically different REM stage. During this dreaming stage [typically occurring about every 90 minutes], more blood is directed to the brain and your brain waves are almost the same as if you were awake. However, you cannot move your muscles. This immobility apparently keeps you from acting out dreams and hurting yourself or others.
        These REM, or dream, cycles get longer each time they occur during the night and appear to be crucial to mental health. In computerlike fashion, the brain sorts through short-term memory storage, deleting unimportant data and retaining what is desired for long-term memory. Abnormally infrequent dream cycles are known to result in emotional difficulties. Insomniacs, for example, spend less time than average in REM sleep, contributing to a vicious downward spiral of increasing anxiety.

       So, what happens when we are regularly deprived [voluntarily or involuntarily] of these repeated cycles, thus creating a sleep debt? If we get fewer consecutive hours of sleep than we need, we won’t get as much of the last and longest REM sleep period, which is vital to mental health. If our sleep patterns become irregular, consisting of a series of naps, we often don’t get to the deep delta sleep that is necessary to mend our bodies. Those in serious debt suffer from shortened attention spans, memory and vocabulary loss, a lessened ability to think analytically, and diminished creativity.

     What triggers the body to demand sleep? A number of factors evidently combine to create a circadian [daily] rhythm, or wake-sleep pattern. Brain chemistry appears to play a role. Also, there is a nucleus of nerve cells located in the brain that evidently helps control the sleep cycle. This “clock” is situated close to where the optic nerves come together. Light thus influences how sleepy we feel. Bright light wakes you up, while darkness induces sleep.

      Your body temperature is also involved. When your temperature is highest –typically midmorning and midevening –you are the most alert. As your body temperature drops, you become increasingly drowsy. Researchers agree that the pattern of wakefulness versus sleepiness varies with individuals