As you doze off, your body starts reducing it’s energy usage. Breathing and heart rate both slow, muscles relax and body temperature lowers. As you move into deep sleep, brain activity slows noticeably. It’s likely that the body does significant work to recuperate in this stage, clearing and sorting and filing away the activity from the day. These initial sleep stages can take anywhere from 30 to 90 minutes to cycle though.
Once you’ve reached deep sleep, your body moves into REM (rapid eye movement) sleep. While most of the rest of your body experiences temporary paralysis, your brain rapidly picks up for a burst of activity and some intense dreaming. While we don’t know the exact mechanisms of biophysical recovery, REM sleep is essential for mental wellness and establishing the neural connections that affect memory and learning. REM should make up more and more of each sleep cycle throughout the night.