Sleep loss may disrupt the brain's ability to ‘unlearn’ fear

A study reveals how getting less sleep than normal may impair the brain’s capacity to regulate fear. The finding helps explain why frequent sleep disturbances make people more prone to anxiety and post-traumatic stress disorder (PTSD).

Sleep plays a crucial role in maintaining mental health. For example, people with insomnia are approximately three times as likely to develop an anxiety disorder compared with those who sleep normally, according to a systematic review of research published in 2019.

Other studies find that people who experience frequent sleep disturbances — a common issue for health workers and military personnel — have a higher risk of PTSD.

Not getting enough rapid eye movement (REM) sleep, which is the sleep stage when most dreaming occurs, seems to be a particularly important factor in this increased risk.

Sleep in general, and REM sleep in particular, are known to play a vital role in “fear extinction.” This is the process of learning where the stimuli previously associated with unpleasant sensations or experiences now become harmless.

A new brain-imaging study that appears in the journal Biological Psychiatry: Cognitive Neuroscience and Neuroimaging shows how sleep loss disrupts the brain’s ability to forget fear-provoking memories the following day.

Three nights in a sleep lab

Sleep researchers led by Anne Germain, Ph.D., at the University of Pittsburgh, PA, and Edward Pace-Schott, Ph.D., at Harvard Medical School and Massachusetts General Hospital in Charlestown, MA, invited 154 volunteers to spend 3 nights in a sleep lab.

On the first night, the researchers allowed them to follow their usual sleep times. But on the second night, the scientists randomly assigned them to one of three groups: normal sleep, sleep restriction, and sleep deprivation.

The normal sleep group was allowed to go to bed and wake up at their usual times, while the research team woke the sleep restriction group after they had slept half their usual amount. The sleep deprivation group were not allowed any sleep at all.

The following morning, the volunteers underwent a standard experimental procedure for fear conditioning and extinction, while the researchers scanned their brains using functional MRI.

The procedure for conditioning fear involved showing subjects three different colors, one at a time, on a screen while they lay in the scanner.

A mild electric shock accompanied the presentation of two of the colors. This taught the participants to associate these colors with being shocked.

To extinguish this fear learning, one of these colors was then presented in the absence of any shock, allowing participants to learn that it was now “safe.

In the evening, the volunteers were again presented with the colors inside the MRI scanner to discover whether they had successfully erased the fear conditioning.

The scans revealed that during the extinction procedure, the brains of those who had slept normally engaged a network of regions called the salience network, which is involved in conditioned fear. They also employed regulatory areas of the prefrontal cortex that inhibit emotions such as fear.

In contrast, in the brains of subjects whose sleep was restricted, the salience network and pain aversion regions were strongly activated at all stages. Their regulatory regions stayed relatively quiet.

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

On their third night in the lab, all the volunteers were allowed to sleep as normal.

The importance of REM sleep

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

Studies have found that REM sleep helps people unlearn fearful memories from the previous day. The new research suggests it is also important for unlearning fear conditioning on the following day.

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

In the evening, when the researchers tested participants’ memories of the fear extinction, the pattern of activity in their brains was similar to that in the brains of subjects who slept normally.

The scientists speculate that a compensatory mechanism may kick in when people are totally sleep-deprived, protecting their brains from fear conditioning.

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

However, the current study suggests that partial sleep deprivation fails to 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 findings may also have implications for exposure therapy for PTSD and phobias, which involves exposing patients to fear-provoking stimuli in a controlled therapeutic setting. They suggest the treatment may not work well after a poor night’s sleep.

The authors note one important limitation of their study was that it tested the effect of a single night of reduced sleep. Chronic sleep deprivation may have different effects on the brain and its ability to unlearn fearful memories.

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