Mental health issues can be squashed during sleep, including chronic stress, anxiety, depression, and panic.

Researchers at the Department of Neurology of the University of Bern and University Hospital Bern found that this process occurs during REM sleep, the time when people have the most intensely emotional dreams.

The study, published in the journal Science, shows how sleep is a vital method of improving mental health, as it dampens negative emotions and reinforces positive ones.

Rapid eye movement (REM or paradoxical) sleep is a unique and mysterious sleep state during which most of the dreams occur together with intense emotional contents. How and why these emotions are reactivated is unclear, however the prefrontal cortex integrates many of these emotions during wakefulness—but they appear to be paradoxically dormant during REM sleep.

The brain seems to favor the discrimination of safety versus danger, but blocks the over-reaction to emotion, in particular danger.

"Our goal was to understand the underlying mechanism and the functions of such a surprising phenomenon," said Professor Antoine Adamantidis, of the Swiss university.

The researchers used mice to reach their conclusion. To do this they exposed some to a sound that they associated with safety and others to a sound they associated with danger.

They then recorded the brain activity of each mouse while they were awake and asleep to work out how emotional memories are transformed during REM sleep.