In a recent experiment, brain scans of people who viewed emotionally provocative pictures and then went to sleep showed that the part of the brain that handles emotions powered down during rapid eye movement, or REM, sleep—the stage in which dreams occur.

What's more, the subjects reported that the images had less of an emotional charge the morning after. This suggests that REM sleep may help us work through difficult events in our lives, the researchers say.

(Read about the mysteries of why we sleep in National Geographic magazine.)

There's already anecdotal evidence for sleep's therapeutic benefits—such as the oft-repeated adage that a person will go to bed and feel better in the morning, Walker said.

And clinical data show