Dreams are the stories the brain tells during sleep—they’re a collection 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—like known people or familiar locations—but they often take on a fantastical feel. While dreams are frequently interesting, and can allow people to act out certain scenarios that would never be possible in real life, they aren’t always positive—negative dreams, referred to as "nightmares," can create feelings of terror, anxiety, or utter despair, and can lead to psychological distress or sleep problems like insomnia.
The big question, however, is why humans dream. Though it’s been discussed and studied for millennia, it remains one of behavioral science's greatest unanswered questions. Researchers have offered many theories—including memory consolidation or emotional regulation—but a unified one remains, well, a pipe dream. Nevertheless, people continue mining their nighttime reveries for clues to their inner lives, for creative insight, and even for premonitions.