Why humans dream remains one of behavioral science’s great unanswered questions. Dreams have a purpose but it may not be to send us messages about self-improvement or the future, as many believe. Instead, many researchers now believe that dreaming mediates memory consolidation and mood regulation, a process a little like overnight therapy. But it’s not a benefit all share equally: People who are sleep deprived also tend to be dream deprived, spending less time dreaming and perhaps not remembering dreams as well.

If dreams were movies, they wouldn’t make a dime. They’re often banal, frequently fleeting and they’re screened for an audience of just one. As for the storyline? You’re in a supermarket, only it’s also Yankee Stadium, shopping with your second-grade teacher until she turns into Ruth Bader Ginsburg. Then you both shoot a bear in the cereal aisle. Somebody call rewrite. But dreams are vastly more complex than that, and if you’ve got a theory that explains them, have at it. The ancient Egyptians thought of dreams as simply a different form of seeing, with trained dreamers serving as seers to help plan battles and make state decisions. The ancient Greeks and Romans believed that dreams were equal parts predictions of future events and visitations by the dead.

A dream is a succession of images, ideas, emotions, and sensations that usually occur involuntarily in the mind during certain stages of sleep. The content and purpose of dreams are not fully understood, although they have been a topic of scientific, philosophical and religious interest throughout recorded history.