Dreaming may help people cope with the pain of difficult memories, research suggests. US researchers found that brain chemicals associated with stress fall during the dream phase of sleep - REM sleep - as the brain processes emotional experiences. This shut down in stress chemistry helps to ease the pain associated with difficult memories, and acts as a type of "overnight therapy," said researchers at University of California, Berkeley. In addition to providing clues into why we dream, the researchers say the study may help to explain some of the symptoms of post-traumatic stress disorder (PTSD), such as recurring nightmares. For people with PTSD, this overnight therapy may not be working as effectively as it should, they said. The researchers divided 35 volunteers into two groups. Both groups were shown 150 images designed to provoke an emotional response, while a MRI scanner measured brain activity. The first group viewed the images in the morning and in the evening, while the second group viewed them in the evening and the following morning after a full night's sleep. The group which viewed the images after a night's sleep reported reduced emotional reactions to the images, compared with the group of non-sleepers. In the group of non-sleepers, the MRI scans also showed less activity in the amygdala, a part of the brain that processes emotions, and more activity in the brain's pre-frontal cortex - the part associated with rational thinking. The researchers also used electroencephalograms to record the electrical activity of the volunteers' brains as they slept. During REM sleep, certain electrical activity patterns decreased, which suggested that levels of stress neuro-chemicals in the brain were reduced. Dr Matthew Walker, who led the study, said: "We know that during REM sleep there is a sharp decrease in levels of norepinephrine, a brain chemical associated with stress. "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."