Dreams are the stories the brain tells during sleep—collections of clips, images, feelings, and memories that involuntarily occur during the REM (rapid eye movement) stage of slumber. Humans typically have multiple dreams per night that grow longer as sleep draws to a close.
It’s hypothesized that everyone dreams, but a small subsection of the population reports that they never remember experiencing dreams.
Dreams typically involve elements from waking lives—known people or familiar locations—but often take on a fantastical feel. Dreams are frequently interesting, and can allow people to act out certain scenarios that would never be possible in real life, but they aren’t always positive—negative dreams, referred to as "nightmares," can create feelings of terror, anxiety, or despair, and can lead to psychological distress or sleep problems like insomnia.
Why humans dream remains one of behavioral science's great unanswered questions. Researchers have offered many theories—including memory consolidation or emotional regulation—but a verifiable one remains elusive.