Getting only half a night's sleep hijacks the brain's ability to unlearn fear-related memories, which might put people at greater risk of conditions such as anxiety or post-traumatic stress disorder (PTSD).

The study, published in the journal Biological Psychiatry: Cognitive Neuroscience and Neuroimaging, provided new insights into how sleep deprivation affects brain function to disrupt fear extinction.

For the findings, the research team studied 150 healthy adults in the sleep laboratory.

One-third of the participants got normal sleep, one third were sleep restricted, so they slept only the first half the night, and one third were sleep-deprived, so they got no sleep at all. In the morning, all the subjects underwent fear conditioning.

"Our team used a three-phase experimental model for the acquisition and overcoming of fearful memories while their brains were scanned using functional magnetic resonance imaging," said study author Anne Germain from the University of Pittsburgh in the US.

In the conditioning paradigm, participants were presented with three colours, two of which were paired with a mild electric shock.

Following this fear conditioning, the participants underwent fear extinction, in which one of the colours was presented without any shocks to learn that it was now "safe."

That evening, participants were tested for their reactivity to the three colours, a measure of their fear extinction recall, or how well they had "unlearned" the threat.

Brain imaging recorded during the tasks showed activation in brain areas associated with emotional regulation, such as the prefrontal cortex, in people who got normal sleep.

But the brain activity looked very different in people with restricted sleep.

"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," the researchers wrote.

Surprisingly, people who got no sleep lacked brain activation in fear-related areas during fear conditioning and extinction.

During the extinction recall 12 hours later, their brain activity looked more similar to those with normal sleep, suggesting that a limited night of sleep may be worse than none at all.

"Our findings suggest that such partially sleep-deprived individuals might be especially vulnerable to fear-related conditions such as post-traumatic stress disorder," Germain noted.