Post by Elisa Guma
What's the science?
One function of sleep is thought to be resolving emotional distress via reactivation and reorganization of neuronal circuits that were activated during the emotional experience. Animal studies have shown that reactivation may occur in the transition period from slow-wave sleep to rapid eye movement (REM) sleep, known as the ‘transition to REM’ sleep phase, whereas memory transformation may occur during REM sleep. To date, no human study has investigated how 'transition to REM' sleep and REM sleep interact to influence emotional memory circuits. This week in Current Biology, Wassing and colleagues set out to investigate whether interruption of transition to REM and REM sleep could prevent adaptation of the limbic system circuitry (neuronal circuits involved in emotional regulation) in humans.
How did they do it?
The authors recruited 29 participants with a wide range of insomnia severity. Participants’ limbic system activity (with a focus on the amygdala) was assessed using functional magnetic resonance imaging while they listened to audio fragments of themselves singing out of tune to get a brain signature of self-conscious emotion (shame and embarrassment). This was performed before, and after sleep in order to assess overnight amygdala adaptation to the emotional stimulus. Brain activity during sleep was recorded using electroencephalography to determine the total duration of REM and 'transition to REM' episodes, as well as the frequency of interruptions of those episodes. This allowed the authors to compare amygdala activity before and after sleep in response to self-conscious emotion, and to correlate its overnight adaptation with the quality of REM and 'transition to REM' episodes.
In a subset of participants (13) who were able to differentiate odours successfully, the initial distressful exposure (listening to themselves sign out-of-tune) was conditioned to a specific odour, whereas a neutral exposure (listening to someone else sing in-tune) was conditioned to a different odour. This allowed the authors to reactivate the negative memory during sleep by presenting the different olfactory cues. The authors assessed the proportion of time that the ‘transition to REM’ and REM episodes coincided with the presentations of olfactory cues.
What did they find?
The authors observed a significant activation of the limbic system while participants experienced self-conscious emotion (out-of-tune singing). They found that amygdala activation decreased after a night of sleep and that this decrease was proportional to the duration of REM sleep, suggesting that REM sleep is critical in decreasing our reactivity to negative emotions. The ‘transition to REM’ duration was not associated with amygdala reactivity, however, longer ‘transition to REM’ episodes boosted the effect of REM sleep duration on overnight adaptation, suggesting that the role of ‘transition to REM’ might be more indirect. Interestingly, they also observed that more REM sleep interruptions were associated with less overnight adaptation in amygdala reactivity.
In the subset of individuals who experienced memory reactivation with the odour conditioned to their own singing stimulus, the authors observed that if the reactivation occurred during restful REM sleep, this actually enhanced the decrease in amygdala reactivity. However, in those participants that had restless, interrupted, REM sleep, this memory reactivation had adverse effects of amygdala reactivity, leading to a smaller overnight decrease in reactivity.
What's the impact?
This study showed that sleep, specifically REM and the preceding 'transition to REM' stage, is critical in decreasing the neural activity associated with a distressing emotion. This can only occur if these stages are not interrupted or compromised. These findings provide further understanding of the role of REM sleep in emotional processing. A better understanding of these underlying mechanisms may help individuals whose REM sleep is chronically interrupted, such as those suffering from insomnia, childhood adversity, depression, anxiety or post-traumatic stress disorder.
Wassing et al. Restless REM sleep impedes overnight amygdala adaptation. Current Biology (2019). Access the original scientific publication here.