In one study from 2009, researchers used fMRIs to test whether lonely brains were more sensitive to threats. Twenty-three participants were placed in an MRI and shown a series of pictures, some of them pleasant, such as money and a rocket lifting off, and others unpleasant, including human conflict. They found that lonely brains respond less positively to pleasant images than non-lonely brains, and more strongly to images of violence and unpleasant social situations. Loneliness spurs the brain into a hyper-vigilant state, unable to relax. The lonely brain doesn’t passively take the world in, but actively interprets it as an unfriendly place.
Hawkley found that lonely individuals take longer to fall asleep, wake up more during the night, and sleep less deeply. “The lonely person’s feeling of not being safe, socially safe, could contribute to disrupted sleep,” she says.