Dreaming Takes the Sting out of
Painful Memories
UC Berkeley researchers have found that during
the dream phase of sleep, also known as REM sleep, our stress chemistry shuts
down and the brain processes emotional experiences and takes the edge off
difficult memories.
The findings offer a compelling explanation for
why people with post-traumatic stress disorder (PTSD), such as war veterans,
have a hard time recovering from distressing experiences and suffer reoccurring
nightmares. They also offer clues into why we dream.
"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," said Matthew Walker, associate professor of psychology and
neuroscience at UC Berkeley and senior author of the study to be published on
Nov. 23, in the journal Current Biology.
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."
The results offer some of the first insights into
the emotional function of Rapid Eye Movement (REM) sleep, which typically takes
up 20 percent of a healthy human's sleeping hours. Previous brain studies
indicate that sleep patterns are disrupted in people with mood disorders such
as PTSD and depression.
While humans spend one-third of their lives
sleeping, there is no scientific consensus on the function of sleep. However,
Walker and his research team have unlocked many of these mysteries linking
sleep to learning, memory and mood regulation. The latest study shows the
importance of the REM dream state.
"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, a doctoral student in psychology at UC Berkeley and lead author of the
study.
Thirty-five healthy young adults participated in
the study. They were divided into two groups, each of whose members 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. The
remaining half viewed the images in the evening and again the next morning
after a full night of sleep.
Those who slept in between image viewings
reported a significant decrease in their emotional reaction to the images. In
addition, MRI scans showed 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.
In addition, the researchers recorded the
electrical brain activity of the participants while they slept, using
electroencephalograms. They 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. "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."
Walker said he was tipped off to the possible
beneficial effects of REM sleep on PTSD patients when a physician at a U.S.
Department of Veterans Affairs hospital in the Seattle area 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 norepinephrine in the brain, thereby creating
a more stress-free brain during REM, reducing nightmares and promoting a better
quality of sleep. This suggested a link between PTSD and REM sleep, Walker
said.
"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."
Other co-authors of the study are UC Berkeley
sleep researchers Justin Yao, Shubir Dutt, Vikram Rao and Jared Saletin.
