Bar graph depicting sleep stages on the y axis and hours of sleep on the x axis, ranging from 0 to 8. The night begins by showing awake at 0 hour. Then, in the first 30 minutes, sleep cycles through the four stages of NREM. The first period of REM sleep occurs after 1 hour and lasts for about 30 minutes. Throughout the night, there are four more periods of REM sleep with cycles of NREM sleep between them. As the night progresses, the periods of REM come closer together and last longer.