Scientists have used scans to shed more light on how the brain deals with the memory of unpleasant or traumatic events during sleep.
The University of California, Berkeley team showed emotional images to volunteers, then scanned them several hours later as they saw them again.
Those allowed to sleep in between showed less activity in the areas of the brain linked to emotion.
Instead, the part of the brain linked to rational thought was more active.
Most people have to deal with traumatic events at some point in their lives, and, for some, these can produce post-traumatic stress disorder (PTSD)， leaving them emotionally disturbed long after the event itself.
There is significant evidence that the 20 percent of sleep in which we dream, also called REM sleep, plays a role in the processing of recent memories, and researchers believe that better understanding of this could eventually help PTSD patients.
Dr Matthew Walker, who led the study, said: "We know that during REM sleep there is a sharp decrease in norepinephrine, a brain chemical associated with stress.
"By reprocessing previous emotional experiences in this neurochemically safe environment of low norepinephrine during REM sleep, we wake up the next day, and those experiences have been softened in their emotional strength.
"We feel better about them, we feel we can cope."