Sleep 'a form of overnight therapy'
Sleep was described by Shakespeare as the "balm of hurt minds" - now new research has shown dreaming can take the sting out of painful memories.
A study has found a sharp drop in a brain chemical associated with stress during REM (rapid eye movement) sleep, or the dream phase of sleep.
Researchers at the University of California, Berkeley, said this "shut down" of stress chemistry allows the brain to process emotional experiences.
The findings shed light on the symptoms of post-traumatic stress disorder, such as recurring nightmares, where this form of "overnight therapy" might not be working effectively, according to the researchers.
"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 Els van der Helm, lead author of the study.
The research, published in the journal Current Biology, involved 35 adults who viewed 150 emotional images, twice and 12 hours apart, while an MRI scanner measured their brain activity.
Half of the participants viewed the images in the morning and again in the evening, staying awake between the two viewings, with the remaining half viewing the images in the evening and again the next morning after a full night of sleep.
Those who slept between image viewings reported a significant decrease in their emotional reaction to the images.
In addition, scans revealed a dramatic reduction in reactivity in the amygdala, a part of the brain that processes emotions, allowing the brain's "rational" prefrontal cortex to regain control of the participants' emotional reactions.
The study also recorded the electrical brain activity of the participants while they slept. During REM 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.