A lot happens in your body while you sleep. When you get your ZZZs, you cycle between REM and non-REM sleep. REM stands for rapid eye movement. During REM sleep, your eyes move quickly in different directions. That doesn't happen during non-REM sleep. First comes non-REM sleep, followed by a shorter period of REM sleep, and then the cycle starts over again. Dreams typically happen during REM sleep.

What Happens During Non-REM Sleep? There are three phases of non-REM sleep. Each stage can last from 5 to 15 minutes. You go through all three phases before reaching REM sleep. Stage 1: Your eyes are closed, but it's easy to wake you up. This phase may last for 5 to 10 minutes. Stage 2: You are in light sleep. Your heart rate slows and your body temperature drops. Your body is getting ready for deep sleep. Stages 3: This is the deep sleep stage. It's harder to rouse you during this stage, and if someone woke you up, you would feel disoriented for a few minutes. During the deep stages of NREM sleep, the body repairs and regrows tissues, builds bone and muscle, and strengthens the immune system. As you get older, you sleep more lightly and get less deep sleep. Aging is also linked to shorter time spans of sleep, although studies show you still need as much sleep as when you were younger.

What Is REM Sleep? Usually, REM sleep happens 90 minutes after you fall asleep. The first period of REM typically lasts 10 minutes. Each of your later REM stages gets longer, and the final one may last up to an hour. Your heart rate and breathing quickens. You can have intense dreams during REM sleep, since your brain is more active. Babies can spend up to 50% of their sleep in the REM stage, compared to only about 20% for adults.