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How does sleep quality affect our ability to repress unwanted memories? Image credit: Drew West/Stocksy.
  • Our brains have an ‘in-built’ ability to restrict intrusive memories, which contributes to the maintenance of our mental health.
  • However, a new study from the University of York and the University of East Anglia in the United Kingdom now suggests that disrupted sleep and sleep loss interferes with that ability, making intrusive memories more persistent.
  • This could contribute to a range of mental health conditions, such as anxiety, depression, and even post-traumatic stress disorder (PTSD).
  • The study also suggests that rapid-eye movement (REM) sleep, where dreaming occurs, may be crucial to the ‘housekeeping’ of the brain by maintaining the ability to keep unwanted memories and thoughts at bay.

It is becoming increasingly obvious that sleep plays a range of crucial roles in helping our bodies maintain various aspects of health, including brain health and mental health.

A new study from the Universities of York and East Anglia in the United Kingdom — whose findings appear in PNAS — now looks at an even more intriguing aspect of the relationship between sleep, the brain, and mental health.

The study found that disrupted sleep affected the brain’s ability to restrict intrusive memories, which could contribute to conditions including anxiety, depression, and even post-traumatic stress disorder (PTSD).

Study co-author Marcus Harrington, PhD, a lecturer in psychology at the University of East Anglia who specializes in sleep, memory, emotion, and mental health, told Medical News Today that he and his colleagues conducted this study because they were keen to find out more about why some people might find it more difficult to overcome traumatic experiences than others.

He told us:

“Most people will have a traumatic experience at some point in their lives, but only a small proportion will go on to develop post-traumatic stress disorder (PTSD). Why do some people recover from trauma effectively whereas others are left with debilitating symptoms? Answering this challenging question is a major goal in clinical psychology. One factor that we know increases the likelihood of developing PTSD in the aftermath of trauma is poor sleep. However, we don’t know what it is about sleep loss that seems to make people vulnerable to PTSD and other mental health problems.”

For this study, the researchers recruited 85 healthy adults ages 18–30 years — of whom 30 were male and the rest female — whom they asked to spend one night in the sleep laboratory. Around half of the cohort (43 participants) had to stay awake all night, while the rest had a healthy night’s sleep.

To see how sleep or the lack thereof might influence the brain’s ability to suppress unwanted memories and thoughts, the researchers had previously asked the participants to undertake an exercise: They had to look at pictures of faces, some of which appeared in the context of negative scenes, such as a car crash.

Following the night of sleep or sleep disruption, all participants had to either recall the scene associated with a specific face or suppress the memory of a scene with a negative association. The researchers used functional magnetic resonance imaging (fMRI) scans to investigate the participants’ brain activity at this time.

When asked to recall unpleasant memories, it became apparent that participants who had undergone a sleepless night had less activation of the right dorsolateral prefrontal cortex region of the brain, which is associated with emotional regulation and the inhibition of intrusive thoughts.

By contrast, the well-rested participants had higher activity in this brain region and less activity in the hippocampus, which is associated with memory recall.

In their study paper, the researchers explained that this suggested that well-rested participants were better able to restrict unwanted memories—a function called “memory suppression.” Conversely, this function was impaired in the brains of participants who had spent a sleepless night.

The fact that sleep deprivation thus appears to affect the ability to suppress intrusive memories and possibly other intrusive thoughts could have important implications for mood disorders like depression and anxiety and conditions like PTSD, they further suggested.

“We have known for a long time that when people are sleep-deprived, they’re less effective at inhibiting external actions, such as pressing a button,” Harrington told MNT, and the current study also confirmed that sleep deprivation negatively affects our control over unwanted memories.

However, the researcher added, “understanding how sleep deprivation affects the subjective experience of trying to keep unwanted memories from intruding into conscious awareness is only half the battle — we were also interested in understanding what goes on in people’s brains when they try to suppress memories under conditions of sleep loss.”

“That’s why we used functional MRI to take images of our participants’ brains as they engaged in memory suppression. The additional insight offered by neuroimaging allows us to create a more holistic picture of how poor sleep might pave the way to mental health problems,” he went on to say.

Commenting on the study results, Caroline Horton, PhD, professor of sleep and cognition at Bishop Grosseteste University in Lincoln, U.K., who was not involved in this research, told MNT:

“This is a high-quality study that will, I suspect, initiate a wave of experimental work exploring links between sleep and inhibition, which is very welcome. The experimental methods used in the study are analogies for more pervasive mental health disorders, and serve as a useful model for thinking about how sleep can help to keep unwanted or damaging emotional material at bay, but does not yet demonstrate precisely how disturbed sleep over longer timeframes can impact on specific mental health conditions, such as PTSD.”

There is plenty of research concerned with the ways in which a good night’s sleep helps support our memory function, due to concerns around cognitive decline risk in an increasingly aging population.

There is also accumulating evidence that getting more deep sleep is important in supporting the formation of new memories.

However, memory suppression also plays a key role in health, particularly mental health.

“In our daily lives, we can often be reminded of unpleasant past experiences. For example, somebody might see a red car driving too quickly and it causes them to automatically retrieve memories about a car accident they were once involved in,” Harrington explained to MNT.

“When this happens, people often engage in memory suppression, which allows them to keep the unwanted memories about the car accident out of mind. If the memory suppression function fails, intrusive thoughts can flood conscious awareness, which may be distracting, distressing, or even debilitating,” he said, speaking of this brain function’s short-term benefits.

He also told us that “memory suppression can also have lasting benefits” when it comes to mental health in the long run.

The researcher went on to note that “suppressing an unwanted memory weakens its underlying memory trace, reducing the likelihood of that memory intruding into consciousness again in the future.”

“Moreover, repeatedly suppressing emotionally negative memories can make those memories seem less unpleasant when they are revisited in the future. These far-reaching benefits of memory suppression are believed to make it a fundamental component of healthy emotion regulation.”

— Marcus Harrington, PhD

“We know a great deal about the integral role that sleep plays in regulating our emotions, ensuring that our responses are proportionate to stimuli and the environment, keeping general anxieties at bay, and allowing us to cope in a wide range of situations,” said Horton.

“Moreover, sleep disturbances are associated with every psychiatric syndrome that you could think of,” she pointed out.

Studies from the past year have added to the evidence surrounding the relationship between mental health and sleep.

One large study, for instance, found that people who go to bed late, past 1 a.m., are more likely to experience mental ill health, including conditions such as depression and anxiety.

While the study did not prove a causal relationship, its authors suggested that later bedtimes may result in less rapid-eye movement (REM) sleep — the type of deep sleep within which most dreams occur, and which helps maintain the brain healthy.

In the current study, the researchers further found that participants who had spent more time in REM sleep later had better activation of the right dorsolateral prefrontal cortex, which more effectively supported memory suppression.

“The dorsolateral prefrontal context, the area of the brain demonstrated to be impaired following a night of sleep deprivation, is interestingly an area that has been shown to be relatively inactive during REM sleep relative to wake,” said Horton.

Work from my own lab has demonstrated that, during dreaming, we experience a range of heightened emotional experiences, likely as a result of this area of the brain effectively turning its activity down, that ordinarily prevents us from thinking about such emotions. Taken together, it is plausible that we experience these emotional scenarios during sleep, when it is safe to do so, leading to a well-regulated ability to engage that inhibition action when we are awake.”

– Caroline Horton, PhD

“When we don’t have the ability to experience those emotional experiences during sleep, then we do so when we are awake – hence the intrusions of memories and experiences that are pervasive, emotional, and likely highly unpleasant,” Horton added.

The issue that arises is that people with diagnosed mental health conditions — such as anxiety and depression — tend to have more difficulty in obtaining uninterrupted sleep. And, since insomnia can be a symptom of poor mental health, this could reinforce a vicious cycle that disrupts crucial brain functions, such as that of memory suppression.

“The relationship between disturbed sleep and mental ill health is almost certainly cyclical,” Horton told us. “It is very difficult to establish definitively whether disturbed sleep causes maladaptive emotional responses, or vice versa, but a range of research approaches indicate a bidirectional relationship, whereby poor sleep may worsen existing maladaptive thinking, and maladaptive thinking leads to disturbed sleep.”

“[P]oor sleep leads to intrusive thoughts that may further obstruct the sleep needed to support recovery, creating a vicious cycle of distressing thought intrusions and chronic sleeplessness, which could plausibly culminate in mental health problems,” Harrington further explained.

The good news is that restoring deep sleep could help redress some of these issues:

“An important finding in our study was that rapid eye movement (REM) sleep was associated with the overnight restoration of the brain mechanisms that drive memory suppression. Sleep researchers are currently developing strategies to enhance REM sleep using methods such as auditory brain stimulation.”

— Marcus Harrington, PhD

Harrington said this could improve therapeutic approaches in the future: “Future studies could investigate whether directly manipulating REM sleep could help people to keep intrusive thoughts out of mind, and if so, whether it can be used in the treatment of mental health problems associated with intrusive thoughts.”

However, he also cautioned that, for now, “this line of research is still in its infancy — patient studies are required to bridge the gap between our research and clinical practice.”

What is most important, Harrington emphasized, is actively working to improve our sleep hygiene now, in order to prevent becoming seriously affected by the disruptions poor sleep causes in our brains.

“Another possible application of our research is in the prevention of mental health problems,” he told MNT. “Recent studies from Professor Michael Anderson’s lab at the Cognition and Brain Sciences Unit in Cambridge suggests that thought suppression is a skill that can be trained, and that such training can benefit mental health.”

“It would be useful to find out if such training works in people with insomnia. If it does, thought suppression training could potentially be used as a therapeutic tool targeted at preventing people with sleep issues from developing mental health problems.”

— Marcus Harrington, PhD

Horton emphasized that “sleeping well is a lifelong habit and, as such, will take a little time to learn and perfect.” She thus advised that we should not feel discouraged if we do not see immediate results when we set out to improve our sleep.

“[I]t’s important to appreciate that great sleep might not appear on night one, and even the best sleepers have bad nights sometimes,” she told us.

With that in mind, the first step toward achieving better sleep is to pay attention to our sleep routines and plan our bedtimes according to our individual needs, the expert advised:

“Make a note of when you yawn, for instance. Are you regularly feeling sleepy at 10.30 p.m.? If so, it would make sense to be in bed by that time! Plan, as best you can, some regular bed- and rise- times, allowing at least 8 hours in bed, which should allow you time to sleep for around 7.5 hours, plus a little more for getting to sleep and getting up. Of course, those times can be adjusted depending on how long you take to get to sleep.”

Then, she added, “There are endless so-called sleep hygiene recommendations available, including refraining from caffeine and/or alcohol, not eating within 3 hours of going to bed, and ensuring your bedroom sleep environment is conducive to relaxing and switching off.”

“All of these are evidence-based and can help individuals,” said Horton, “but alone, they may only have a small impact on sleep quality.”

The key is, once again, “customizing” our habits to our individual requirements, and making small, gradual changes: “Think about your own needs and, rather than making lots of changes at once, try to make small changes and see how they go. We are more likely to stick with a new behaviour if it can align with our values and existing habits.”

“Finally, consistency is key,” Horton stressed. “It may be tempting to stay up late on a Friday night and sleep in at weekends, but that confuses your body and brain, meaning it will be extra difficult to rise early in the week.”

Evidence shows that catching up at weekends does not compensate for the damage caused by sleeping too little in the week, and it also prevents a regular routine from developing,” she concluded.