Your Body Does Incredible Things When You Aren't Awake

Sleep is anything but a waste of time. While many people think of catching Zzs as the body "shutting off" until morning, nodding off actually activates a complicated process that helps you feel rested and healthy the next day. Scientists divide sleep into four stages -- each full sleep cycle takes about 90 minutes to complete, meaning we cycle through roughly five rotations during 7.5 hours between the sheets. The amount of time spent in each stage of sleep varies by cycle, with more deep sleep taking place earlier in the night and more dreaming sleep in the second half of the night. In honor of Sleep Awareness Week, here's a closer look at some of the amazingly complex things your body does every night.

Stage one: The earliest stage of sleep is one between wakefulness and slumber. It's easy to be awoken from stage one sleep and, if you are, you might even feel like you haven't slept at all. Your eyes move slowly back and forth as your muscles relax and sometimes even twitch, a phenomenon called a hypnagogic jerk.

Stage two: Philip Gehrman, Ph.D., assistant professor of psychiatry at the University of Pennsylvania, dubs stage two sleep "average sleep" -- it's not too deep, not too light, and it's where you spend about half the night. As you move into stage two sleep, you lose a sense of your surroundings and your body temp drops, according to the National Sleep Foundation. Brain waves are slow (with some rapid bursts) and your heart rate and blood pressure slow down and regulate. That means, for much of the night, your heart and vascular system are getting a much-needed rest, which might help to explain the many cardiovascular benefits of shuteye.

Stage three: "This is our deepest sleep," Gehrman says of stage three (which used to be separated into stages three and four). "It's also called our slow-wave sleep because our brain waves are these slow, high-amplitude waves." During this restorative stage of sleep, blood pressure drops, breathing slows down, blood flow moves to the muscles and tissue is repaired, according to the National Sleep Foundation. Hormones, such as the human growth hormone, are secreted at this time as well. Stage three is also where parasomnias, such as sleep walking, talking or eating, take place. Since this is the deepest sleep (and the one that makes you feel well-rested and energetic the next day), it's also the hardest stage to be woken from, Gehrman says. About 20 percent of the night is spent in deep sleep, and it mostly happens in the first half of the night.