How sleep aids emotional processing

 Researchers from the University of Bern's Department of Neurology and the University Hospital Bern discovered how the brain triages thoughts during dream sleep to solidify pleasant feelings while reducing negative ones. The research highlights the relevance of sleep in psychological health and suggests novel therapeutic approaches.

Rapid eye movement (REM) sleep, also known as paradoxical sleep, is a perplexing sleep condition in which most dreams come in tandem with significant emotional content. It's unclear why and how these feelings are triggered. During waking, the prefrontal cortex processes many of these emotions, but during REM sleep, it appears to be quiet. "Our goal was to understand the underlying mechanism and roles of such a surprise phenomena," says Prof. Antoine Adamantidis of the University of Bern's Department of Biomedical Research (DBMR) and the Inselspital, University Hospital of Bern's Department of Neurology.

Animals' existence depends on their ability to process emotions, notably the ability to discern between danger and safety. Excessively negative emotions in humans, like fear reactions and anxiety states, can lead to pathological conditions such as Post-Traumatic Stress Disorder (PTSD). In Europe, almost 15% of the populace suffers from chronic anxiety and serious mental disease. During REM sleep, the study group led by Antoine Adamantidis is now revealing how the brain helps to sustain pleasant emotions while weakening severely negative or traumatic feelings. The results of this research were published in journal Science.

A Dual Mechanism

The mice were initially trained to distinguish between auditory stimuli relating to safety and those linked with hazard (aversive stimuli). During sleep-wake cycles, the firing of neurons in mice's brains was recorded. The scientists were able to scan distinct parts of a cell in this way and figure out how emotional memories change during REM sleep.

Neurons have a cell body (soma) that combines information from dendrites (inputs) and send impulses towards other neurons through their axons (outputs). Cell somas are kept quiet when their dendrites are stimulated, according to the findings. "This entails a dissociation of the two cellular compartments," Adamantidis continues, "in other words, the soma is wide asleep and the dendrites are wide awake." This decoupling is significant because the dendrites' high activity allows for the storing of both threat and safety feelings, whereas the soma's inhibitions entirely block the circuit's output during REM sleep. In other words, the brain prioritizes safety versus danger discrimination in the dendrites, but blocks over-reaction to emotion, particularly danger.

A survival advantage

The coexistence of both methods, according to the researchers, is helpful to the organisms' stability and survival: "This bi-directional mechanism is crucial to optimise the discrimination between dangerous and safe signals," explains Mattia Aime of the DBMR, the study's first author. Anxiety disorders can result if this discernment is lacking in humans and overwhelming fear emotions are generated. The findings are particularly pertinent to pathological diseases like post-traumatic stress disorder, in which trauma is over-consolidated in the prefrontal cortex during sleep, day after day.

Sleep medication breakthrough

These findings pave the way for a better understanding of human emotion processing during sleep, as well as potential therapeutic targets for maladaptive traumatic memory processing, like Post Traumatic Stress Disorders (PTSD) and their initial sleep-dependent consolidation. Acute and chronic stress, worry, despair, anxiety, or even anhedonia, the incapacity to feel pleasure, are all conditions that could be linked to this somatodendritic dissociation during sleep. The University of Bern and the Inselspital, Bern University Hospital, have long been interested in sleep research and sleep medicine. "We believe that our findings will be of interest to both patients and the general public," Adamantidis says.