What are dreams for? A handful of theories predominate. Sigmund Freud famously contended that they reveal hidden truths and wishes.  More recent research suggests that they may help us process intense emotions,  or perhaps sort through and consolidate memories,  or make sense of random neuron activity,  or rehearse responses to threatening situations.  Others argue that dreams have no evolutionary function, but simply dramatize personal concerns. 
Despite being largely unsupported by evidence, Freud’s view maintains a strong following around the world. Researchers found that students in the U.S., South Korea, and India were much more likely to say that dreams reveal hidden truths than to endorse better-substantiated theories.  Relatedly, people put great stock in their dreams: In the same study, respondents said that dreaming about a plane crash would cause them more anxiety than an official warning about a terrorist attack.
Even if dreams can’t foretell the future, they seem to expose our shared fascinations. The majority of dreams occur during REM sleep cycles, of which the average person has four or five a night. Eight percent of dreams are about sex, a rate that holds for both women and men—though women are twice as likely as men to have sexual dreams about a public figure, while men are twice as likely to dream about multiple partners.  Anxiety is also rife: A study of Canadian university students found the most common dream topics, apart from sex, to be school, falling, being chased, and arriving too late for something. 
For all the commonalities dreams exhibit, they vary across time—people who grew up watching black-and-white TV are more likely to dream in black and white —and culture. A 1958 study determined that compared with Japanese people, Americans dreamed more about being locked up, losing a loved one, finding money, being inappropriately dressed or nude, or encountering an insane person. Japanese people were more likely to dream about school, trying repeatedly to do something, being paralyzed with fear, or “wild, violent beasts.”  (For their part, beasts almost certainly have nightmares too: Just about all mammals are thought to dream, as are birds, some lizards, and—unique among invertebrates—cuttlefish.  The dreamiest member of the animal kingdom is the platypus, which logs up to eight hours of REM sleep a day. )
If human dreams sound bleak, bear in mind that even negative ones can have positive effects. In a study of students taking a French medical-school entrance exam, 60 percent of the dreams they had beforehand involved a problem with the exam, such as being late or leaving an answer blank. But those who reported dreams about the exam, even bad ones, did better on it than those who didn’t. 
So the next time you dream about an education-related sexual experience in which you are both falling and being chased, don’t worry: It’s probably totally meaningless. Then again, your brain might be practicing so you’ll be ready if such an event ever comes to pass.
 Sigmund Freud, The Interpretation of Dreams (1913)
 Van der Helm et al., “REM Sleep Depotentiates Amygdala Activity to Previous Emotional Experiences” (Current Biology, Dec. 2011)
 Wamsley and Stickgold, “Memory, Sleep and Dreaming: Experiencing Consolidation” (Sleep Medicine Clinics, March 2011)
 Hobson and McCarley, “The Brain as a Dream State Generator” (American Journal of Psychiatry, Dec. 1977)
 Antti Revonsuo, “The Reinterpretation of Dreams” (Behavioral and Brain Sciences, Dec. 2000)
 G. William Domhoff, “A New Neurocognitive Theory of Dreams” (Dreaming, March 2001)
 Morewedge and Norton, “When Dreaming Is Believing” (Journal of Personality and Social Psychology, Feb. 2009)
 Antonio Zadra, “Sex Dreams” (Sleep, 2007)
 Nielsen et al., “The Typical Dreams of Canadian University Students” (Dreaming, Dec. 2003)
 Eva Murzyn, “Do We Only Dream in Colour?” (Consciousness and Cognition, Dec. 2008)
 Griffith et al., “The Universality of Typical Dreams” (American Anthropologist, Dec. 1958)
 Frank et al., “A Preliminary Study of Sleep-Like States in the Cuttlefish” (PLOS One, June 2012)
 Siegel et al., “Monotremes and the Evolution of Rapid Eye Movement Sleep” (Philosophical Transactions of the Royal Society B: Biological Sciences, July 1998)
 Arnulf et al., “Will Students Pass a Competitive Exam That They Failed in Their Dreams?” (Consciousness and Cognition, Oct. 2014)
This article appears in the April 2019 print edition with the headline “Study of Studies: Bad Dreams Are Good.”
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