How we process emotions during sleep may change our mental health
By Sarah Berry
A longstanding piece of advice when it comes to processing difficult things and making big decisions is to “sleep on it”. It’s widely acknowledged that the dawn of a new day tends to provide us with a new perspective.
As American author E. Joseph Cossman said: “The best bridge between despair and hope is a good nights sleep.” But why? Well, researchers are one step closer to understanding.
During REM sleep, our brain mimics its awake state.Credit: iStock
The discovery of different sleep stages in the 1950s led to new insights about their function: during slow wave sleep, for instance, our brain performs a deep-clean of itself, which researchers believe may be critical in protecting against dementia and general cognitive decline.
During the surreal period of rapid eye movement (REM) sleep, our brain mimics its awake state, but our body is paralysed, so we don’t act out our dreams. Noradrenaline (the “fight or flight” neurotransmitter) is switched off – for the only time each day – allowing us to start processing the experiences of our waking hours, effectively separating the emotion from the information.
This means we retain the information we need about a distressing incident, so we can respond to (or avoid) the experience in future. But as the emotion is processed (in part during our dreamtime), the intensity of the feeling slowly subsides.
Despite these insights, there are still gaps in the knowledge about the underlying mechanisms. This is important as there are implications for treating insomnia, as well as a range of mental health conditions.
Dreamtime processing in a neurochemical bath
For a new study, the Woolcock Institute’s Dr Rick Wassing analysed 244 studies spanning 20 years of research to understand how sleep helps us to deal with our emotional memories and what happens when that processing phase of sleep is disrupted, as a result of insomnia and conditions such as depression, anxiety and post-traumatic stress disorder (which are strongly linked to poor sleep).
Published in Nature Reviews Neuroscience, Wassing and his team found that the way certain neurochemicals are regulated during REM sleep is crucial for the processing of emotional memories and our long-term mental health.
Both noradrenaline and serotonin, which helps us to logically evaluate the world around us as well as to regulate happiness, switch off, while acetylcholine (a neurotransmitter that plays a role in memory, learning, attention) peaks.
This process allows a transformation to occur, inducing plasticity in the parts of the brain which consolidate new experiences into memory and weakening connections that stop us from feeling the initial distress.
“The absence of noradrenaline during REM sleep is critical for ‘uncoupling’ the memory trace connections with the amygdala thereby reducing the autonomic fight-or-flight response,” Wassing explains.
However, researchers including Wassing believe that noradrenaline does not properly switch off in people with insomnia, those who suffer PTSD and other mental health conditions. This leads to restless or disrupted REM and may mean traumatic experiences are never properly processed, nor are they detached from the emotion of the initial experience. This means the original intensity of the distress can be re-triggered over and over again.
What it means for sleep and mental health treatments
These findings have several implications. The research and the insights are so new that current medications do not target these mechanisms and may explain their limited efficacy.
“But we can’t just point the finger to suboptimal treatment standards without providing an alternative, and that’s what we’re working on,” Wassing says, noting that his team is currently conducting studies on new medications and looking at their impact on emotional processing.
As for current approaches, most of which reduce REM sleep, they suggest that no REM may be better than restless REM and could explain why REM sleep-reducing antidepressants do help some people.
“Patients with insomnia have more restless REM sleep compared with normal sleepers, have impaired overnight adaptation to emotional distress, and are at risk of mental health problems,” the authors write. “Although our focus was on insomnia, the maladaptive sleep may well be applicable to other sleep disorders such as sleep apnoea.”
Monash University clinical neuroscience professor Sean Drummond, who is also a member of the Sleep Health Foundation, says the paper provides a compelling theory.
He is however sceptical of the suggestion that no REM may be better than fragmented REM.
“After all, if REM sleep plays a fundamentally important role in the healthy processing of emotions and new emotional memories, then a complete lack of REM sleep does not allow for that function at all,” he says, but adds that his own research supports the idea that restless REM may be worse.
It’s confusing, admits Ron Grunstein, a professor of sleep medicine at the University of Sydney and head of the NHMRC Centre for Integrated Research and Understanding of Sleep.
“There is a need for REM sleep,” Grunstein says. “Some people with sleep apnoea just have it in REM and often have mood and cognitive problems, which are quite significant.”
In the long-term, when people are deprived of REM, there is a rebound effect and the brain hungers for more of it during sleep. But, whether trying to suppress REM sleep is helpful in the short term, he’s unsure. “People are different,” he says. “There are some limits to knowledge.”
What we do know is that poor mental health and poor sleep go together, says the Sleep Health Foundation’s Professor Dorothy Bruck.
“They are two sides of the same coin,” she says. “The more you can have good REM sleep, the less emotionally distressing your memories are.” Doing more research to understand how we can all improve our REM, Bruck says, can only be a good thing.
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