We know that sleep is critical for our mental health, but a new study shows that a certain phase of sleep is important for our emotional health as well. In the study, published in the journal Science, Swiss researchers examined the brain activity of mice during wakefulness, REM, and non-REM sleep. They found that during REM sleep, neurons in the prefrontal cortex at the front of the brain appear to reinforce positive emotions while also dampening our most negative and traumatic ones. REM sleep – short for rapid eye movement – is the phase when we do most of our dreaming. The researchers believe that REM sleep plays a role in post-traumatic stress disorder, anxiety, and depression.