Summary: Restless REM sleep disrupts the adjustment of memory traces during sleep. This may increase negative emotions associated with mental health disorders.
Upset by something unpleasant? We have all been there. Fortunately, it also passes. A new day, a new beginning. At least: if you have restful REM sleep. Researchers at the Netherlands Institute for Neuroscience discovered why you will be better able to bear tomorrow what you are distressed about today. And why that can go wrong.
Siren of the brain
Something frightening or unpleasant does not go unnoticed. In our brain, the so-called limbic circuit of cells and connections immediately becomes active. First and foremost, such experiences activate the amygdala. This nucleus of brain cells located deep in the brain can be regarded as the siren of the brain: attention! In order for the brain to function properly, the siren must also be switched off again. For this, a restful REM sleep, the part of the sleep with the most vivid dreams, turns out to be essential.
The researchers placed their participants in an MRI scanner in the evening and presented a specific odor while they made them feel upset. The brain scans showed how the amygdala became active. The participants then spent the night in the sleep lab, while the activity of their sleeping brain was measured with EEG, and the specific odor was presented again on occasion. The next morning, the researchers tried to upset their volunteers again, in exactly the same way as the night before. But now they did not succeed so well in doing this. Brain circuits had adapted overnight; the siren of the brain no longer went off. The amygdala responded much less, especially in those who had had a lot of restful REM sleep and where meanwhile exposed to the specific odor.
However, among the participants were also people with restless REM sleep. Things went surprisingly different for them. Brain circuits had not adapted well overnight: the siren of the brain continued to sound the next morning. And while the nocturnal exposure to the odor helped people with restful REM sleep adapt, the same exposure only made things worse for people with restless REM sleep.
Neuronal connections weaken and strengthen
During sleep, ‘memory traces’ of experiences from the past day are spontaneously played back, like a movie. Among all remnants of the day, a specific memory trace can be activated by presenting the same odor as the one that was present during the experience while awake. Meanwhile, memory traces are adjusted during sleep: some connections between brain cells are strengthened, others are weakened. Restless REM sleep disturbs these nocturnal adjustments, which are essential for recovery and adaptation to distress.
The findings were published on 11 July in the leading journal Current Biology. The finding can be of great importance for about two-thirds of all people with a mental disorder, as both restless REM sleep and a hyperactive amygdala are the hallmarks of post-traumatic stress disorder (PTSD), anxiety disorders, depression, and insomnia. People with PTSD carry their traumatic experience to the next day: people with an anxiety disorder take their greatest fear with them, people with depression their despair, and people with chronic insomnia their tension. Authors Rick Wassing, Frans Schalkwijk, and Eus van Someren predict that treatment of restless REM sleep could transdiagnostically help to process emotional memories overnight and give them a better place in the brain.
Rick Wassing – KNAW
The image is in the public domain.
Original Research: Open access
“Restless REM Sleep Impedes Overnight Amygdala Adaptation”. Rick Wassing, Oti Lakbila-Kamal, Jennifer R. Ramautar, Diederick Stoffers, Frans Schalkwijk, Eus J.W. Van Someren.
Current Biology. doi:10.1016/j.cub.2019.06.034
Restless REM Sleep Impedes Overnight Amygdala Adaptation
• A novel self-conscious emotional experience elicits amygdala activation
• Overnight amygdala adaptation is proportional to the duration of sound REM sleep
• Preceding sleep spindles increase the benefit of sound REM sleep
• Overnight amygdala adaptation fails proportionally to the restlessness of REM sleep
Animal studies show that insufficient silencing of the locus coeruleus (LC) during REM sleep impairs sleep-related brain plasticity. Restless REM sleep, a characteristic of several psychiatric disorders, likely reflects insufficient LC silencing. We investigated whether endogenous REM sleep interruptions interfere with overnight reorganization of limbic circuits in human volunteers with a wide range of insomnia severity, from no insomnia complaints to fulfilling community-sample criteria for insomnia disorder. We induced a self-conscious emotion during two functional MRI sessions and recorded sleep EEG in between. Amygdala reactivity decreased overnight in proportion to the total duration of consolidated REM sleep. Restless REM sleep, in contrast, impeded overnight amygdala adaptation. Using targeted memory reactivation with odors tagged to the self-conscious emotional stimulus, we could experimentally enhance both the favorable effect of consolidated REM sleep and the unfavorable effect of restless REM sleep. The findings reveal a maladaptive type of sleep, providing a target for interventions in mental disorders characterized by restless REM sleep.