How do nightmares happen when we sleep?

Nightmares: Dreams in general occur during REM sleep as a way for the mind to process the many stimuli that are experienced during the day. Nightmares can occur when there is stress, a major change or a death. In some people nightmares can be associated with drug use, lack of sleep and certain prescription medications. Depression and Anxiety disorders have also been linked to nightmares.

Nightmares: Nightmares are intense and frightening dreams that tend to occur during REM (rapid eye movement) sleep. Nightmares are more common in the last part of the sleep cycle. They typically are not rooted in reality so they should be ignored.