Researchers at the University of California (UC), Berkeley, have discovered that during REM or the dream phase sleep, our body’s stress chemistry shuts down while the brain processes emotional experiences and eases the pain in difficult memories. They suggest their findings, reported online in the journal Current Biology on Wednesday, offer a compelling explanation for why people with post-traumatic stress disorder (PTSD) have recurring nightmares and a hard time recovering from distressing experiences.
Senior author Matthew Walker, associate professor of psychology and neuroscience at UC Berkeley, says in a media statement:
“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.”
Their findings may also offer clues about why we dream at all, and about the emotional function of Rapid Eye Movement (REM) sleep, which comprises around 20% of the time a healthy person spends asleep.
Previous studies have shown that people with PTSD, depression and other mood disorders have disrupted sleep patterns.
People with PTSD experience flashbacks: for instance if the traumatic event that caused their condition was a bomb explosion, then the sudden noise of a car backfiring can trigger a flashback where they experience the same visceral reactions, feelings and body sensations, as they did in the original event.
Walker suggests the reason flashbacks persist is “because the emotion has not been properly stripped away from the memory during sleep”.
Lead author Els van der Helm, a doctoral student in psychology at UC Berkeley, explains:
“During REM sleep, memories are being reactivated, put in perspective and connected and integrated, but in a state where stress neurochemicals are beneficially suppressed.”
For their study, the researchers put 35 young healthy adults into two groups. The first group viewed a set of 150 emotionally arousing images twice: first in the morning, and then 12 hours later in the evening. The second group also viewed the same images twice, but the first time was in the evening and the second time was 12 hours later in the morning, after a full night’s sleep.
The researchers used an MRI scanner to measure participants’ brain activity while they viewed the images, and in the group that had the overnight sleep, they also used electroencephalograms to record the electrical brain activity during sleep.
The results showed a significant reduction in emotional reaction to the images between the first and second viewing in the group that slept overnight between viewings. The MRI scans of this group also showed a dramatic reduction in reactivity of the amygdala, the part of the brain that processes emotions. This reduction allowed the “rational” prefrontal cortex part of the brain to regain control of the participants’ emotional reactions, said the researchers.
When they examined the electroencephalogram recordings of the group that had slept between viewings, they found during REM dream sleep, certain patterns of electrical activity diminished.
Walker says we already know that REM sleep is accompanied by a sharp fall in levels of norepinephrine, a brain chemical associated with stress. So he and his colleagues suggest that the fall in stress chemicals in the brain is what soothes the emotional reactions that arise in the processing of memories of the previous day’s experiences.
“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,” says Walker.
He and his colleagues write in their conclusion:
“… we demonstrate that REM sleep physiology is associated with an overnight dissipation of amygdala activity in response to previous emotional experiences, altering functional connectivity and reducing next-day subjective emotionality.”
Walker became interested in exploring this when a doctor who works at a US Department of Veterans Affairs hospital in Seattle told him about a blood pressure drug with a curious side effect: it seemed to prevent recurring nightmares in PTSD patients.
On further investigation, Walker discovered that the generic blood pressure suppresses norepinephrine in the brain, thus making the brain more “stress-free” during REM sleep, with the effect of reducing nightmares and increasing quality of sleep.
Walker says this meant there must be a link between PTSD and REM sleep.
“This study can help explain the mysteries of why these medications help some PTSD patients and their symptoms as well as their sleep,” he says, adding that “it may also unlock new treatment avenues regarding sleep and mental illness”.
Written by Catharine Paddock PhD