Why do we have nightmares?

Nightmares are disturbing dreams that provoke fear, anxiety, or sadness, usually happening during the REM stage of sleep late in the night when dreaming is most intense. They often wake the sleeper, who can typically remember the dream vividly. Common triggers include stress and anxiety, traumatic experiences, sleep deprivation, fever, and some medications or substances. One influential idea, the threat-simulation theory, suggests that nightmares evolved to let the brain safely rehearse responses to dangers, sharpening survival instincts. Nightmares are especially common in children and usually decrease with age. Occasional bad dreams are normal, but frequent, distressing nightmares that disrupt sleep may be a sign of nightmare disorder or an underlying condition such as post-traumatic stress.