Never go to sleep thinking dark thoughts

A good night’s sleep can solidify our memory of a nasty or traumatic event, making it harder to suppress, a new study in Nature Communications suggests.

The work, by Yunzhe Liu, a neuroscientist at Beijing Normal University in China, and colleagues has implications in treating conditions such as post-traumatic stress disorder.

“The ability to suppress unwanted emotional memories is crucial for human mental health,” they write.

“Through consolidation over time, emotional memories often become resistant to change. However, how consolidation impacts the effectiveness of emotional memory suppression is still unknown.”

We’re all too aware that traumatic memories hold a certain level of vividness compared to neutral experiences. While many of us can suppress these thoughts, some patients with psychiatric disorders – such as post-traumatic stress disorder – find it difficult to block out negative memories.

To figure out how sleep is linked to this process of memory suppression, the researchers set about exploring where negative memories sit in the brain, and how that changes after a good night’s sleep.

To scan for these memories, the team used functional magnetic resonance imaging to observe the brain activity of 73 male college students.

The students were shown images of faces and asked to associate them with images that elicit negative emotions in people. The participants were shown the faces again after 30 minutes, and then after 24 hours, and asked to try to suppress the associated negative images.

The participants’ brain activity showed that neural circuits involved in the suppression process moved from the hippocampus to specific neocortical regions after 24 hours, suggesting a shift, or consolidation, had taken place.

The researchers think it’s this shift that makes memories harder to suppress over time.

When the test was repeated using neutral images – pictures that didn’t elicit such a dramatic emotional response – the results were similar, suggesting this consolidation process applies to all memory.

“These findings demonstrate rapid changes in emotional memory organisation with overnight consolidation,” the researchers write.

They are still unsure whether the results point to the time elapsed, rather than sleep, and another study using an awake control group would help to clarify this.

But the study shows the significance of memory consolidation and suppression in a clinical setting when treating patients with post-traumatic stress disorder and other memory-related disorders.