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The results show that during nighttime dreaming, also known as REM sleep, the brain processes emotional experiences in a "safe" environment, or one in which stress chemicals are low. This processing may take the emotional edge off of difficult memories, the researchers said.
"We wake up the next day, and those experiences have been softened in their emotional strength," said study researcher Matthew Walker, an associate professor of psychology and neuroscience at the University of California, Berkeley."We feel better about them, we feel we can cope."
The findings may explain why people with post-traumatic stress disorder (PTSD), such as war veterans, have a hard time recovering from painful experiences, the researchers said.
For people with PTSD, Walker said, this overnight therapy may not be working effectively, so when a "flashback is triggered by, say, a car backfiring, they relive the whole visceral experience once again, because the emotion has not been properly stripped away from the memory during sleep," Walker said.
Previous studies indicate that sleep patterns are disrupted in people with mood disorders such as PTSD and depression, Walker said.
Good dreams
The study involved 35 healthy adults, divided into two groups. Each group viewed 150 emotional images — including a man holding a gun, a shark and a snake about to bite — while they had their brains scanned using magnetic resonance imaging (MRI).
Half of the participants viewed the images in the morning and again in the evening, staying awake between the two viewings. The other half viewed the images in the evening and again the next morning, after a full night of sleep.
Those who slept between viewings reported a significant decrease in their emotional reaction to the images. In addition, MRI scans showed a dramatic reduction in activity in the amygdala, a part of the brain that processes emotions — this reduction would allow the brain's "rational" prefrontal cortex to regain control of the participants' emotional reactions, the researchers said.
The researchers also recorded the electrical activity in participants' brains while they slept. During REM dream sleep, that the researchers saw reduced levels of stress chemicals in the brain, which could be helping to soothe emotional reactions to the previous day's experiences.
"During REM sleep, memories are being reactivated, put in perspective and connected and integrated, but in a state where stress neurochemicals are beneficially suppressed," said study researcher Els van der Helm, a doctoral student in psychology at UC Berkeley.
Effect on PTSD
Walker said he was tipped off to the possible beneficial effects of REM sleep when a physician at a U.S. Department of Veterans Affairs hospital told him of a blood pressure drug that was inadvertently preventing reoccurring nightmares in PTSD patients.
It turns out that the generic blood pressure drug had a side effect of suppressing stress chemicals in the brain, thereby creating a more stress-free brain during REM. This may reduce nightmares and promote a better quality of sleep.
"This study can help explain the mysteries of why these medications help some PTSD patients and their symptoms as well as their sleep," Walker said. "It may also unlock new treatment avenues regarding sleep and mental illness."
The study is published today (Nov. 23) in the journal Current Biology.
Pass it on: Dreaming may help to make bad memories feel less painful.
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