"Wishes suppressed during the day assert themselves in dreams," Sigmund Freud wrote more than a century ago. Now new research provides evidence suggesting that not just wishes but all kinds of thoughts we bar from our minds while awake reappear when we sleep.

Harvard University psychologist Daniel M. Wegner and his colleagues tested this notion on 330 college students. Participants were asked to think of a personal acquaintance and then spend five minutes engaging in one of three mental exercises before going to sleep. Members of the first group suppressed their thoughts about the person, those in the second focused their thoughts on the person, and those in the third were instructed to think freely about anything that came to mind. In the morning, the subjects wrote down what they had dreamed. The researchers found that, overall, the students dreamed about their target person more frequently than did people who had not specifically thought of someone before turning in for the night. But the chosen acquaintances most often appeared in the dreams of those subjects who had made a conscious effort to block them from their minds, the team reports in a forthcoming issue of the journal Psychological Science.

"Maybe this is why students dream of sleeping through an important exam, why actors dream of going blank on stage, and why truckers dream of driving off the road," Wegner offers. "Dreams are where our thoughts go when we try to put the thoughts out of mind."

In explanation Wegner and his collaborators note that the thoughts we deem unwelcome might resurface in dreams because the prefrontal cortex (the part of the brain that is responsible for planning and mental control) is less active during REM sleep, thus diminishing the brains ability to keep them at bay. Looking forward, the investigators plan to examine unique sets of unwanted thoughts that arise in the dreams of people with distinct psychological conditions, such as anxiety disorders and phobias. "It could be that dreams are diagnostic," Wegner suggests. "They may be important indicators of what we try not to think of during the day." --Alla Katsnelson