Another reason not to go to bed angry

Sleeping on negative memories makes them harder to forget.

When we go to sleep, our brains reorganize the new information we’ve absorbed during the day, moving it from short-term to long-term memory. That’s a good thing if you’ve just studied for a test, but sometimes we have experiences we’d rather forget. New research demonstrates that these same mechanisms make it harder to suppress negative or traumatic memories after sleeping.

“Through previous studies on cognitive control, we know that people can suppress memories voluntarily,” says lead author Yunzhe Liu, a PhD student at the Wellcome Trust Centre for Neuroimaging, “but we did not know whether it is better to suppress memories before or after sleep.” Identifying this window could be important for treating psychiatric disorders like PTSD, says Liu.

Liu and his coauthors decided to put it to the test. They showed 73 male college students disturbing images—among them badly injured people, corpses, crying children, and ongoing crimes like kidnapping and robbery—alongside neutral faces, creating an association between the image and the face. Later that day, the students were shown the face alone and asked to suppress the memory of the associated image. When they repeated the task after a full night’s sleep, the students had a harder time suppressing their memory of the image than they did the day before.

“Overnight consolidation makes aversive memory more resistant to suppression by promoting hippocampal-neocortical reorganization of the memory,” explains Liu, who is under the supervision of Prof. Ray Dolan. Brain images taken during the task showed that the neural circuits used to suppress the memory shifted after sleep, from the hippocampus to the cortex. It is this change that seemingly makes the memory harder to suppress.

Although the study only looked at men due to gender differences in emotion regulation and cognitive control, Liu suspects the results would be the same for women. However, he cautions that well-designed experiments are needed to know for sure.

The results of this study alone do not have definitive implications for the treatment of PTSD. However, they do suggest that “reconsolidation,” a process in which old memories are modified and re-stored, may be a promising technique for suppressing traumatic memories in those who suffer from related disorders. As for more mundane things we’d like to forget, Liu has a simple piece of advice: “Don’t sleep on your anger.”

Featured image courtesy of Tracy Abildskov