The drawing above records our stage of sleep, during the night. It’s called a “Hypnogram”.
“Light sleep” means we’re sleeping, but we are not paralyzed and we wake more easily. “Deep sleep” means we’re paralyzed and doing the “work” of sleep. Both Slow wave sleep and REM sleep are considered “deep sleep”. REM is marked in red. Slow wave sleep is marked in blue.
Here’s how to read the hypnogram: Read from left to right. Start on the left with awake at 11:00pm. At 11:05 first light sleep, then second stage light sleep. Then at about 11:15 pm comes the first stage of deep sleep, or Slow Wave Sleep (SWS)(blue).  We do deep sleep in blocks of time, usually no more than an hour. We’re paralyzed in deep sleep and don’t want to stay paralyzed in one position for too long. At 12:10 am we go back up to light sleep again, (not paralyzed) change position, and go back into deep sleep again, this time REM at 12:15 ( in red) until 12:30, then back to light sleep briefly and back to SWS at about 12:45 am (blue). We do more SWS at the beginning of the night and more REM sleep toward the end of the night. Most of the REM sleep happens between 3-6 am. Usually, if you wake in a dream you were in REM sleep. REM = Rapid Eye Movement. The sleep trackers like the FitBit and the Oura ring call SWS ”deep sleep” and REM “REM”. The sleep doctors call both SWS and REM sleep ”deep sleep” because we’re paralyzed in those two phases.