Each sleep phase serves a specific physiological purpose. The primary function of both our light-sleep and deep-sleep phases is to have a regenerative effect on various processes in the body.

We reach the first of our deep-sleep stages, stage 3, after approximately 20 to 30 minutes, and the second, stage 4, after about 45 minutes. At that point, the body is completely relaxed, and we are more or less completely disconnected from reality. If you want to wake someone from deep sleep, you need to make a lot of noise or shake them quite hard. Waking someone from stage 4 is almost impossible—a bit like trying to wake a hibernating bear in the middle of winter. This is the most restful part of the night’s sleep. Muscular activity decreases even further, and our eyes do not move. Stages 3 and 4 make up about 20 percent of our time asleep, but this proportion decreases as we get older. Here’s how you can combat America’s sleep-deprivation crisis—and get more shut-eye.

During non-REM sleep, the body repairs itself, builds bone and muscle, and bolsters the immune system. As you get older, this non-REM sleep decreases significantly—from around two hours per night when you’re under 30 to possibly just 30 minutes when you’re over 65. And during both deep sleep and REM sleep, the brain works to process the impressions and memories of the day. For example, if you get an adequate amount of sleep the night before an exam, including several deep-sleep and REM phases, you’ll be better able to recall the material you’ve studied.

A review of research published in the journal Biology of Mood & Anxiety Disorders in 2015 also indicates that getting a sufficient amount of REM sleep prior to a traumatic or fear-inducing experience, as well as in the early stages of a trauma’s aftermath, may make a person less likely to develop PTSD. The bottom line: Sleep is incredibly important for a variety of reasons, some of which we’re still learning about. That’s why you won’t want to miss these easy ways to sleep better.

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