Insomnia prevents the brain from letting go of fear

To monitor this process, the researchers used the galvanic skin response — a change in the electrical properties of the skin due to the activity of the sweat glands. This is a recognized marker of stress: the greater the anxiety, the more intensely the palms sweat. The authors built two mathematical models that translate this signal into an assessment of nervous arousal: one took into account only sudden spikes, while the other also considered the background level of tension between them.

The experiment involved 24 people with insomnia and 24 people with normal sleep. First, the participants were shown a visual stimulus that was repeatedly paired with a mild electric shock, thereby forming a conditioned fear. The next day, the same stimulus was presented without the unpleasant accompaniment: the participants had to unlearn and learn that it was no longer dangerous.

Only the model that took into account the background tension captured the key difference. From the fear‑formation phase to the unlearning phase, the arousal indicator increased in people with insomnia, while it gradually decreased in those who slept well. In other words, the bodies of healthy participants learned to calm down as the stimulus lost its danger, while in those suffering from insomnia, tension, on the contrary, increased even after the threat had already disappeared. There were other differences: in people with insomnia, arousal was lower during the formation of fear and higher on the second day, when the memory of fear and the memory of safety were tested again, but these differences did not reach statistical significance.

According to the authors, the overall picture is as follows: in people with insomnia, the safety memory either failed to fully consolidate on the first day or did not persist until the second day. As a result, it turned out to be too weak to counteract the original fear. “Our study shows that insomnia is not just about poor sleep,” summarizes lead author Tianyu Hwang. “It is directly linked to the body’s ability to learn and remember that the threat has passed.”

The results also have practical significance. It is noted that overexcitement is considered a risk factor for the development of insomnia, and it likely hinders the successful treatment of fears: methods such as exposure therapy, in which a person is gradually and safely introduced to a frightening situation, may work better for those who get enough sleep.

The authors also see an applied perspective, as wearable devices that track sweating are increasingly being integrated into gadgets, and in the future, such data may help assess sleep and stress levels in everyday life.