What Happens When We Sleep?

Answer:

Shortly after we fall asleep, the body becomes very relaxed. Our heartbeat, breathing and digestive system slow down. Most of our muscles become loose and floppy. This state is called deep sleep.

About an hour later, our muscles start to twitch. Breathing becomes faster and shallower, and our eyes flick about, even though the eyelids stay closed. This state is called REM (rapid eye movement) sleep.

After around 20 minutes, the body relaxes again into deep sleep. These changes from deep to REM sleep happen every hour or two through the night. The deep sleep gradually becomes lighter each time, until eventually we wake up.