When do nightmares occur?

Sleep of a healthy person is divided into two phases. The first one is a sleep of NREM (non-rapid eye movement). The second one is sleep of REM (rapid eye movement). While REM sleep isn’t typically divided into stages, then NREM, because of the participation of the slow waves, is divided into four stages. Normally NREM takes 80-100 minutes, then is followed by REM sleep, which takes about 15 minutes. These phases are repeated after each other cyclically during the night. Adult man will experience from 4 to 6 such cyclical phases in his sleep. During both, REM and NREM, you can experience dreams, however most often they are formed in the REM stage, which is identified with dreaming and therefore nightmares.