A new study finds that sleep may play a role in how the brain processes emotionally-charged memories, which could have important implications in how we understand how post-traumatic stress disorder (PTSD) works in the brain. Though researchers haven’t quite reached a consensus on why we sleep, we do know that sleep helps our brains rebound after a long day. Also among its functions are sleep's ability to help consolidate memories, enhance cognitive function and boost decision-making.

Reactivity of the amygdala – a brain region important in emotion and stress – was reduced during the second viewing in the participants who had slept.

To see how sleep might affect how memories are processed, researchers showed participants 150 emotional images (like an open-mouthed shark) twice, separated by a 12-hour period. Half of the participants were shown the images in the morning, and then again at night. The other half were shown the images at night, and after a full night’s sleep, were presented with the second set of images. The team asked the participants about their subjective responses to the images, and looked at their brain activity with fMRI and EEG.

The participants who slept between the two viewings reported a less pronounced reaction to the second set of images. The researchers saw that the reactivity of the amygdala – a brain region important in emotion and stress – was reduced during the second viewing in the participants who had slept. Electroencephalograms or EEGs verified that reduced levels of the stress neurochemical norepinephrine were linked to less emotional reactivity the following morning.

“We know that during REM sleep there is a sharp decrease in levels of norepinephrine, a brain chemical associated with stress," said study author Matthew Walker. "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."

Altered sleep patterns have been linked to mood disorders like depression, anxiety, and PTSD. REM sleep in particular seems to be disrupted in PTSD patients. Walker says he got the idea for the study when he heard that a side effect of a blood pressure drug was decreased incidence of nightmares in PTSD patients: the blood pressure drug is known to reduce norepinephrine levels in the brain. "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 REM “therapy” will be important both for PTSD patients who are coping with extremely stressful memories and for regular people coping with stressful situations in their daily lives. "The dream stage of sleep, based on its unique neurochemical composition,” says Walker, “provides us with a form of overnight therapy, a soothing balm that removes the sharp edges from the prior day's emotional experiences.”

The study took place at the University of California at Berkeley, and was published in the November 23, 2011 online edition of Current Biology.