The adage about not going to bed angry may have some scientific merit.
According to a small study published Tuesday, sleep reinforces negative memories in the brain. So if angry or disturbing thoughts are on your mind when you drift off to sleep, you may have more difficulty suppressing them later.
At least, that’s the theory.
“In our opinion, yes, there is certain merit in this age-old advice,” Yunzhe Liu, the study’s lead author, told Guardian reporter Hannah Devlin. “We would suggest to first resolve [an] argument before going to bed. Don’t sleep on your anger.”
The study, which appears in the journal Nature Communications, was conducted at Beijing Normal University, although Liu is currently a Ph.D. student in neuroscience at University College London.
Both helpful and harmful
As Liu and his colleagues explain in the background section of their paper, during sleep the brain reorganizes and consolidates newly acquired information, shifting it from short-term to long-term memory.
A good night’s sleep can therefore be helpful if you’ve just been studying for an exam of some kind (and plenty of research has shown that). But, conversely, sleep may be harmful if you want to forget an anger-filled — or a traumatic — memory.
As the researchers also point out, past research has shown that people can suppress memories voluntarily, although very little is known about how sleep influences that ability.
For their current study, Liu and his colleagues asked 18 male university students, aged 20 to 24, to memorize 26 images of “neutral” faces — ones not associate with either positive or negative emotions. Each of the faces was paired with a disturbing image, such as a badly injured accident victim or a distressed and crying child. The purpose of the pairing was to get the students to associate each face with an upsetting image.
A separate group of 30 students were recruited as a “control group.” For them, the faces were paired with neutral, non-emotional images.
The following day, after a night’s sleep, the students were shown the faces alone and asked to suppress their memories of the associated images.
The experiment was then repeated with another 26 pairs of faces and disturbing images. This time, only 30 minutes passed before the students were asked to suppress the images.
Finally, the students were shown all 52 faces and asked to recall each of the paired images that went with them. The purpose was to see if sleep made the task of forgetting memories — particularly negative images — more difficult.
It did. The students struggled more with suppressing their memories of the negative images after they had slept, a finding that suggests sleep helps consolidate such memories in the brain.
Magnetic resonance imaging (MRI) scans taken of the students’ brains while they were trying to suppress the disturbing images revealed a potential explanation for why the process was more difficult after a night’s sleep.
The MRI scans showed that when the students’ memories of the images were only 30 minutes old, they could be found in the hippocampus, a part of the brain known to be associated with the formation of new memories and learning (and also with emotion).
But when the memories had been “slept on,” they appeared to have become more distributed across the brain, particularly across the cortex, where complex mental processes take place.
This finding suggests, say the researchers, that the memories had been transferred during sleep into the brain’s long-term storage.
Liu and his colleagues confirmed these observations in a parallel study involve 25 additional male students.
Limitations and implications
The study has many important limitations. Most notably, it involved very few people, and those who did take part in the study were all young male college students living in China. It’s not clear if the results would be the same in a larger and more diverse group of volunteers.
The researchers also acknowledge that their observed effects might simply be a reflection of the passage of time (24 hours vs. 30 minutes) rather than of overnight sleep.
The findings do support, however, other research that suggests that the distribution of memories within the brain changes significantly during sleep as those memories are moved from short-term into long-term storage.
And if the study’s findings hold up, they may have clinical uses, such as in the treatment of people with post-traumatic stress disorder (PTSD), say Liu and his colleagues. An inability to forget a traumatic event is a hallmark of that condition.
“Sleep deprivation immediately after traumatic experiences may prevent traumatic memories from being consolidated into stabilized representations and thus provide the opportunity to block the formation of traumatic memories,” the researchers write.
That’s a big jump to make from this little study. A smaller message might be more appropriate: Try to keep your late-night hours anger-free. These days, that probably involves avoiding the news.
FMI: You can read the study in full on the Nature Communications website.