In the light of the recent tragedies in Paris, the process of post-traumatic healing has been on our minds a lot this week. Although acts of violence are clearly by themselves in a unique category of things that can go wrong, I am sure we have all suffered some other form of powerful loss, maybe a bad breakup or rejection letter from a job opportunity. Strong negative emotions are simply a natural part of life. I am sure we have all had that experience of going to bed with a gloomy heart, maybe even with tears in our eyes.
Sometimes when we wake up the next morning, the gloom magically disappears. We wake up to see the sunlight coming in through the window and everything feels fine again. But other times, the sunlight goes unnoticed: we wake up in a state of gloom that is just as bad as before we went to bed. So what happens when we sleep? How can sleeping and dreaming affect our emotional state when we wake up?
In 2009, a study was conducted at the University of California, Berkeley, to determine the effect of sleeping on memory. This study was conducted on 31 people who did not suffer from any psychiatric or sleep disorder. These people were kept on a strict diet and daily routine during and for 1 week prior to the experiment. They were not allowed to drink anything containing alcohol or caffeine. During the experiment, the subjects were divided into 2 groups: a nap group and no-nap group. Both groups were shown a set of 240 pictures, 120 of which were designed to trigger negative emotions and 120 of which were emotion-neutral.
Both groups were allowed to study these pictures before they were given a memory test. In the test, the subjects were shown a mixture of pictures, half of which were the pictures they had seen before and another half new. The subjects were tasked to figure out which pictures they had seen and which ones were new. To prepare for the test, subjects were allowed to study half of the original pictures (120 of them) for four hours before the test and another half 15 minutes before the test. The nap group was also allowed to take a nap of up to 90 minutes between the two study sessions. This experiment was, therefore, designed to answer three questions: 1) how napping affects memory, 2) how the recall of emotional imagery is different from nonemotional imagery, and 3) how study time (4 hours vs. 15 minutes before the test) affects recall.
Interestingly, there was no difference in the recall of emotional or nonemotional imagery for the non-nap group. Whether the pictures were studied four hours before the test or 15 minutes before the test, the recall rate was the same. On the other hand, there was a significant improvement in the recall of emotional imagery for the nap group. The members of this group actually recalled the images they saw before their nap better than the images they saw after. The recall of nonemotional imagery, on the other hand, remained the same, indicating that sleeping is a way to consolidate emotional memory.
Now this mechanism of improved retrieval makes a lot of sense from an evolutionary perspective. In many ways, it is important to our survival that we remember bad experiences very well, even if they are the bad experiences we observe other people having (like those we see in pictures). This likely allows us to avoid the bad consequences we observe in other people.
But what about the intensity of the emotions? How does sleep affect this? A follow-up study conducted at Berkeley in 2011 examined this question directly. Like the previous experiment, the participants of this study were broken up into two groups: a sleep group and a non-sleep group. Both groups were allowed to see a set of pictures like the ones used in the 2009 experiment. But this time instead of just trying to remember which ones they had seen, the participants were asked to rate the emotional intensity. The participants did this rating twice, separated by an interval of 12 hours. The sleep group was allowed to have a night of sleep during this interval while the non-sleep group just spent a normal day without sleeping.
Interestingly, the sleep group consistently rated the pictures as having less intense emotional content on the second viewing. The non-sleep group, on the other hand, gave very similar intensity ratings for both viewings. This essentially means the sleeping seems to have decreased the emotional intensity of the pictures. Like the first experiment, this study also looked at the consolidation of emotional memory and again confirmed that the sleep group remembered the emotional pictures better.
So what does this mean? It appears our brain does two things to emotional memory when we sleep.
First, it consolidates it so we remember it better. Second, it seems to decrease the intensity of the emotion. What this effectively means is that sleeping might actually be good for overcoming intense negative experiences. This mechanism makes evolutionary sense too because there usually is no advantage for us to dwell too long on bad memories such as breakups or rejections. Life has to go on.
Huang Xu is an undergraduate student in the School of Life Sciences at Fudan University.
Justin Fendos is a Ph.D. from Yale and a professor at Dongseo University in Busan. He is also the associate director of the Tan School of Genetics at Fudan University, Shanghai, and a National Academy of Sciences Teaching Fellow.