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Sleep loss can interrupt the ability of the brain to ‘unlearn’ fear

A study demonstrates how having less sleep than normal can affect the ability of the brain to suppress fear. The result helps understand why people are more susceptible to anxiety and post-traumatic stress disorder (PTSD) because of regular sleep disturbances.

In sustaining mental health, sleep plays a crucial role. According to a systematic analysis of studies published in 2019, people with insomnia, for example, are nearly three times more likely to develop an anxiety disorder compared to those who sleep normally.

Other studies show that people who experience recurrent sleep problems have a higher risk of PTSD, a common concern for health professionals and military personnel.

The very important factor in this increased risk appears to be not having enough rapid eye movement (REM) sleep, which is the sleep stage where most dreaming occurs.

In general, sleep, and REM sleep in particular, are considered to play a crucial role in the “extinction of fear.” This is the learning process where the stimuli previously associated with negative feelings or interactions are now harmless.

In the journal Biological Psychiatry: Cognitive Neuroscience and Neuroimaging, a recent brain-imaging study reveals how sleep deprivation disrupts the capacity of the brain to forget fear-provoking memories the following day.

Three nights in a sleep lab

At Harvard Medical School and Massachusetts General Hospital in Charlestown, MA, sleep researchers, led by Anne Germain, Ph.D., at the University of Pittsburgh, PA, and Edward Pace-Schott, Ph.D., invited 154 volunteers to spend three nights in a sleep lab.

The researchers allowed them to follow their normal sleep cycles on the first night. The scientists, however, randomly assigned them to one of three groups on the second night: normal sleep, sleep restriction, and sleep deprivation.

The regular sleep group was able to go to bed and wake up at their usual hours, while after sleeping half their usual number, the research team woke the sleep restriction group. The sleep deprivation group were not allowed any sleep at all.

The next morning, the participants underwent a typical experimental protocol for conditioning fear and extinction, while the researchers used functional MRI to scan their brains.

The technique for conditioning fear involved displaying three distinct colors to subjects, one at a time on a screen while they were in the scanner.

The appearance of two of the colors was followed by a mild electric shock. It taught the respondents to equate these colors with being surprised.

In the absence of any shock, one of these colors was then presented to remove this fear of learning, encouraging participants to learn that it was now “safe.”

In the evening, the volunteers were again faced with the colors inside the MRI scanner to find out if the fear conditioning had been effectively removed.

The scans showed that the brains of those who had slept normally engaged a network of regions called the salience network, which is involved in conditioned fear, during the extinction procedure. They also used prefrontal cortex regulatory areas that suppress emotions like fear.

In comparison, the salience network and regions of pain aversion were strongly activated at all stages in the brains of subjects whose sleep was controlled. Their regulatory regions have remained relatively quiet.

“We found that among the three groups, those who had only had half a night’s sleep displayed the most fear-related activity in brain regions and the least activity in emotion control-related areas,” says Dr. Pace-Schott.

All the volunteers were granted permission to sleep as usual on their third night in the lab.

The importance of REM sleep

The researchers speculate that only sleeping the first half of the night deprives a person of most of their REM sleep, which occurs mostly towards the end of a normal period of sleep.

Studies have shown that REM sleep helps people unlearn the previous day’s frightening memories. The new study indicates that it is also necessary on the following day to unlearn fear conditioning.

The researchers were shocked to discover that during the experiment’s fear conditioning and extinction phases, fear-related regions in the brains of participants who were completely deprived of sleep did not work.

In the evening, when the researchers checked the memories of participants of the extinction of fear, the activity pattern in their brains was identical to that in the brains of subjects who usually slept.

The scientists speculate that when people are completely sleep-deprived, a compensatory mechanism will kick in, shielding their brains from conditioning terror.

They write that a similar mechanism can explain why through sleep deprivation therapy, some people with depression experience a temporary easing of their symptoms.

The current research, however, indicates that partial deprivation of sleep does not activate this protective mechanism.

“Medical workers and soldiers often have curtailed or interrupted sleep rather than missing an entire night’s sleep […] Our findings suggest that such partially sleep-deprived individuals might be especially vulnerable to fear-related conditions such as PTSD.”

– Dr. Pace-Schott

The results may also have implications for PTSD and phobia exposure therapy, which includes exposing patients in a controlled therapeutic setting to fear-provoking stimuli. After a bad night’s sleep, they say the treatment could not work well.

One significant limitation of their study the authors note, was that it tested the influence of a single night of reduced sleep. Deprivation of chronic sleep may have numerous effects on the brain and its ability to unlearn fearful memories.

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