Senior author Matthew Walker, associate professor of psychology and neuroscience at University of California, Berkeley, found that during the dream phase of sleep -- REM sleep -- the stress chemistry shuts down and the brain processes emotional experiences and takes the painful edge off difficult memories.
"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," Walker said in a statement.
The findings, published in the journal Current Biology, offer some insight into the emotional function of REM sleep, which typically takes up 20 percent of a healthy human's sleeping hours. Previous brain studies indicate sleep patterns are disrupted in people with mood disorders such as PTSD and depression, Walker explained.
However, for people with PTSD, the overnight therapy may not be working effectively.
Using electroencephalograms, the researchers found that during REM dream sleep, certain electrical activity patterns decreased, showing that reduced levels of stress neurochemicals in the brain soothed emotional reactions to the previous day's experiences.
"We know that during REM sleep there is a sharp decrease in levels of norepinephrine, a brain chemical associated with stress," Walker said.
A generic blood pressure drug has a side effect of suppressing norepinephrine in the brain, thereby creating a more stress-free brain during REM, reducing nightmares and promoting a better quality of sleep, Walker said.