Sleeping on a negative thought helps preserve it, a recent study concluded. Participants were shown images. Some of the images were positive, others negative. Twelve hours later, those who’d been asleep had better recognition of the images. The negative images and the positive ones.

Emotional reaction to the negative images was greatly diminished over time when participants stayed awake. But for people who slept, the reaction to the negative images was just as strong as when they’d first viewed them. Researchers also linked a longer time spent in REM sleep with preserving emotional reactions to the images. The study was unable to connect REM sleep with recognizing the images.

The study was conducted by the Department of Psychology and Neuroscience and Behavior Program at the University of Massachusetts. It appeared in the January issue of The Journal of Neuroscience.