Researchers have identified the sleep mechanism that enables the brain to strengthen emotional memories.
They also found that a commonly prescribed sleep aid heightens the brain’s remembrance of and response to negative memories.
Dr. Sara Mednick from the University of Riverside and her colleagues found that a sleep condition known as sleep spindles — bursts of brain activity that last for a second or less during a specific stage of sleep — are vital for emotional memory.
In earlier research, Mednick demonstrated the vital role that sleep spindles play in transferring memories from short-term to long-term in the hippocampus.
The drug zolpidem (brand names include Ambien and others) was found to enhance the process, a discovery that could lead to new sleep therapies to improve memory for aging adults and for those with dementia, Alzheimer’s and schizophrenia. It was the first study to show that sleep could be manipulated with medication to improve memory.
“We know that sleep spindles are involved in declarative memory — explicit information we recall about the world, such as places, people and events,” she explained.
But until now, researchers did not know that sleep spindles were involved in emotional memory; they had been focusing on rapid eye movement (REM) sleep instead.
Using two commonly prescribed sleep aids — zolpidem and sodium oxybate (Xyrem) — the researchers were able to tease apart the effects of sleep spindles and rapid eye movement (REM) sleep on the recall of emotional memories. They determined that sleep spindles, not REM, affect emotional memory.
For the study, the researchers gave zolpidem, sodium oxybate and a placebo to 28 men and women between the ages of 18 and 39 who were normal sleepers. They waited several days between doses to allow the medications to leave their bodies.
The participants were shown images known to induce positive or negative responses for one second before and after taking supervised naps. After taking zolpidem, participants recalled more images that had negative or highly arousing content, which also suggests that the brain may lean more strongly toward consolidation of negative memories, Mednick said.
“I was surprised by the specificity of the results, that the emotional memory improvement was specifically for the negative and high-arousal memories, and the ramifications of these results for people with anxiety disorders and PTSD,” she remarked. “These are people who already have heightened memory for negative and high-arousal memories. Sleep drugs might be improving their memories for things they don’t want to remember.”
The study, published in the Journal of Cognitive Neuroscience, has implications for people suffering from insomnia related to post traumatic stress disorder (PTSD) and other anxiety disorders, and who are also prescribed zolpidem as a sleep aid.
Currently, the U.S. Air Force uses zolpidem as one of the prescribed “no-go pills” to help flight crews calm down after using stimulants to stay awake during long missions, the researchers noted in the study.
“In light of the present results, it would be worthwhile to investigate whether the administration of benzodiazepine-like drugs may be increasing the retention of highly arousing and negative memories, which would have a countertherapeutic effect,” they wrote. “Further research on the relationship between hypnotics and emotional mood disorders would seem to be in order.”