Deep sleep can provide much needed rest after a difficult day, but a new study suggests it can also help decrease the emotional intensity of painful experiences.

Researchers at the University of California at Berkeley found that the more time spent in REM sleep, or the dream phase of sleep, may diminish the activity of stress-related chemicals in the brain.

“The dream stage of sleep, based on its unique neurochemical composition, provides us with a form of overnight therapy, a soothing balm that removes the sharp edges from the prior day’s emotional experiences,” said Matthew Walker, a  co-author of the study and an associate professor of psychology and neuroscience at UC Berkeley in a statement.

Their findings, the authors said, could help explain why people who have  with post-traumatic disorder  experience recurring nightmares.  One of the hallmarks of the disorder is less time spent in REM sleep.  As a result, the researchers believe they don’t experience the same emotional blunting brought on by adequate REM sleep.  REM sleep normally makes up about 20 percent of normal sleep hours.

In the study, 35 healthy adults were split into two groups.  Each group looked at 150 emotional images two different times, 12 hours apart, and an MRI measured brain activity.

Half of the participants stayed awake between each viewing, and the other half got a full night’s  in between each viewing.

Those who slept had a less emotional reaction the second time they looked at the images, and the MRI showed less activity in the amygdala, the emotion-processing part of the brain.

Tests that measure brain activity while the participants slept indicated decreased activity of stress-related chemicals, which had a calming effect.

“We know that during REM sleep there is a sharp decrease in levels of norepinephrine, a brain chemical associated with stress,” Walker said. “By reprocessing previous emotional experiences in this neuro-chemically 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.”