A typical night’s sleep
A typical night of sleep consists of cycling through non-REM sleep. We progress through stage I, II, III and IV of non-REM sleep. We revisit stage III and II of non-REM sleep. We then enter REM sleep. After a brief period in REM sleep, we begin the cycle again, starting with the non-REM stage. The pattern repeats throughout the night. One complete cycle of non-REM and REM sleep takes about 90 minutes. But notice from figure 4.4 that as the night progresses we spend less time in slow-wave sleep and more time in REM sleep. This means that the body-restoring functions of slow-wave sleep take place early on, during the first few cycles of sleep. After these early cycles, we spend longer in REM sleep. So if you are not getting enough sleep, you will miss out on the longest period of REM sleep. On average, we spend roughly 90 minutes of that time in REM sleep. That means each night you spend approximately 90 minutes having REM dreams.