Transcript: You spend a third of your life sleeping-or at least you should! So what goes on while you're snuggling under the covers? Most of us think of sleep as “dead time,” yet it's actually an active state during which the brain and body regenerate. For this reason, a good night's sleep is essential to a good day's productivity. During sleep, the body repeatedly cycles through four stages of non rapid-eye movement sleep, or NREM, and one stage of rapid-eye movement sleep, or REM. Each of the four stages of NREM sleep can last from 5 to 15 minutes. During Stage 1 of NREM sleep, you can be awakened very easily, and, if you are, you may feel like you haven't slept at all. During this first NREM stage, many people experience a feeling of falling, which can cause a sudden muscle contraction, known as hypnic myoclonia. Often, hypnic myoclonia will cause an abrupt awakening. When the body enters Stage 2 of NREM sleep, the heart rate slows and body temperature drops. Muscles tighten and then relax again as you prepare to enter deep sleep.  Stages 3 and 4 of NREM sleep are also known as slow-wave, or delta sleep, although Stage 4 is more intense. If aroused during these stages, you may feel briefly disoriented before awakening fully. During the delta sleep stages of NREM, the body repairs and regenerates energy, builds bone and muscle, and strengthens the immune system. After the body cycles through its first four stages of NREM sleep (generally 90 minutes after sleep onset), it enters its first course of REM sleep. The first period of REM lasts for about 10 minutes. Then, you begin the cycle again, passing through the 4 stages of NREM before re-entering REM sleep. Each subsequent stage of REM lengthens until the last segment, which may last up to an hour. During REM sleep, the eyes move rapidly in different directions, hence the term “rapid-eye movement.” Heart rate and respiration also speed up and become erratic. Dreaming only occurs during REM, as a result of the heightened brain activity in this stage. Paradoxically, during this time your muscles are paralyzed.  To be properly restored and regenerated, the body must repeatedly cycle through all of these sleep stages, usually for about 7 to 8 hours a night in adults. If you have trouble falling asleep, or can't sleep through the night, if you wake up too early, or if you are tired during the day, you may have a sleep disorder. Common sleep disorders include standard insomnia, sleep apnea, narcolepsy, restless leg syndrome, nightmares and sleepwalking. Because the body needs sleep to properly restore and repair itself, it's vital to get your uninterrupted eight hours. If you're not sleeping as you should, please see your doctor.