The sleeping brain goes through repeated cycles of NREM sleep and REM sleep. Non-REM sleep or NREM sleep is divided into light sleep and deep sleep. REM sleep or Rapid Eye Movement sleep usually occurs 45 to 50 minutes after falling asleep. Vivid dreams occur in REM sleep.The first episode is brief and  subsequent cycles of REM sleep will be even longer in duration.Typically, adults go through 4 cycles of NREM and REM sleep each night.