'Do not go to bed angry' is a common advice for couples who want to improve their relationship, and it may have a scientific backing. Researchers have found out that negative, emotionally-charged memories, like the ones that follow an argument, were harder to suppress after a good night's sleep.
It is thought that sleeping is required to consolidate a memory, no matter the memory type. Learning and remembering new information is much more difficult to do if you don't get enough sleep. A range of studies have shown that even a brief nap can boost learning, memory and creativity and this effect is even more important after a full night of sleep.
However, it is not known whether the consolidation that happens during sleep can make it harder to erase a bad memory on the long term. When people do not want to recall something – for instance a fight with their loved-ones – they might find it harder to do so after sleeping on it. In the new study published in the journal Nature Communications, scientists have tested this hypothesis.
Led by Yunzhe Liu from Beijing University, the team recruited 73 male college students to perform a series of memory suppression tasks. The first part of the study involved training them to remember associations between pairs of images of faces and aversive scenes, either 24 hours or 30 minutes before performing brain scans. This means that the study subjects formed negative memories associated with specific faces, some time before subsequent experiments.
The scientists then tested the participants' brain activity with functional MRI scans, while asking them try and suppress the negative memories induced in the first part of the experiment. This task was repeated on two consecutive days, between which all participants reported having had a good night's sleep.
The scientists found out that on the second day, after sleeping, the participants were less likely to succeed in suppressing negative memories. Brain scans indicated a change in patterns after the night of sleep – the part of the brain involved in memory suppression shifted from the hippocampus to the cortex. This appeared to make the memories more difficult to suppress, suggesting that anger, sadness or trauma being consolidated by sleeping throughout the night.
Helping with PTSD
The scientists say their findings could be useful to understand and treat conditions such as post-traumatic stress disorder (PTSD). Memories of traumatic events are usually vivid and long-lasting and failing to suppress them is an important feature of PTSD.
Over time, these memories often become more resistant to suppression. Better making sense of the role of sleeping in the consolidation of these memories, like this new study tries to do, can thus lead to a better understanding of why they are so hard to get rid of – offering new insights into how best to treat it.