Sleeping on a problem makes them more upsetting.
Unpleasant experiences stay more upsetting if we sleep immediately after the event than if we digest the memory while still awake, researchers have found.
The study contradicts previous papers which claim that sleeping helps us take the edge off negative emotions and view them from a more rational perspective.
Dr Rebecca Spencer, who led the study, said: “We found that if you see something disturbing, let’s say an accident scene, and then you have a flashback or you’re asked to look at a picture of the same scene later, your emotional response is greatly reduced, that is you’ll find the scene far less upsetting if you stayed awake after the original event than if you slept.”
The findings make sense from an evolutionary perspective, she said, because retaining vivd memories of negative events would have taught our ancestors to avoid repeating them in future.
They could also be significant for people suffering from post-traumatic stress disorder or eyewitnesses in court cases, she added.
The researchers, from the University of Massachusetts Amherst, showed 106 young adults a series of pictures, some of which were designed to be upsetting, and asked participants to rate their emotional response to them.
After a 12-hour gap they were shown another set of pictures, some of which were the same, and asked to give fresh emotional feedback on each image and state whether or not they had seen it before.
Participants who slept overnight between viewings were better at recognising images they had seen and retained stronger negative feelings about them compared with those who saw the pictures in the morning and again the same evening.
Dr Spencer said the new study provides the first evidence that sleep preserves not only our memory of emotional events but also our sensitivity to them.
She said: “Some previous studies have looked at just memory itself, not how emotional reactivity changes. When they looked at how our responsiveness changes they have a different and limited measure.
“There have been a few studies which look at one or the other but this is the first to look at them side by side.”