Sleep and Emotion Have Strongly Related Brain Mechanisms

Researchers at University of Bern have identified how sleep consolidates the storage of positive emotions and dampens the consolidation of negative ones.

That healthy sleep is critical for mental well-being and emotion regulation seems evident to me on the basis of personal experience. But it is good to see the importance of sleep in mental health confirmed by scientific studies that also suggest new therapeutic strategies.

Most dreams with intense emotional content occur in a sleep phase called REM (Rapid Eye Movement). Studies have shown that the brain integrates many of these emotions during wakefulness but not during REM sleep.

"Our goal was to understand the underlying mechanism and the functions of such a surprising phenomenon," says research leader Antoine Adamantidis in a press release issued by University of Bern.

A study is published in Science. It provides insights into how the brain helps to reinforce positive emotions and weaken strongly negative or traumatic emotions during REM sleep.

The researchers experimented with laboratory mice trained to recognize sounds associated with safety and other sounds associated with danger. Monitoring and recording the activity of neurons in the mice's brains allowed the researchers to determine how emotional memories are transformed during REM sleep.

The body of a neuron, called soma, integrates information coming from input channels called dendrites and sends signals to other neurons via output channels called axons. The researchers found that somas are kept silent while their dendrites are activated during REM sleep. "This means a decoupling of the two cellular compartments, in other words soma wide asleep and dendrites wide awake," explains Adamantidis.

This decoupling is important because the strong activity of the dendrites allows the encoding of both danger and safety emotions, while the inhibitions of the soma completely block the output of the circuit during REM sleep. In other words, the brain favours the discrimination of safety versus danger in the dendrites. But it blocks the over-reaction to emotion, in particular danger. 

"This bi-directional mechanism is essential to optimize the discrimination between dangerous and safe signals," says researcher Mattia Aime. It seems plausible that evolution has developed this optimization mechanism to favor the survival of all animals in an often dangerous world.

In humans, impairments of this balancing mechanism can lead to excessive fear reactions, anxiety disorders, and post-traumatic stress disorders. In such cases, traumatic emotions are consolidated too much.

These findings pave the way to a better understanding of the processing of emotions during sleep in humans. And they open new perspectives for sleep medicine. In Europe, roughly 15% of the population is affected by persistent anxiety and severe mental illness.

It is hoped that breakthroughs in sleep medicine could not only help patients who suffer from severe mental conditions, but also improve the mental well-being of everyone. "We hope that our findings will not only be of interest to the patients, but also to the broad public," concludes Adamantidis.