Loneliness is more than a bad feeling. It harms our health, our ability to perform, and our sense of fulfillment. When we were hunter-gatherers, having trusted relationships increased our chances of survival. When we were separated from the members of our tribe, we were in grave danger, which triggered a stress state in our body. Over thousands of years, this stress response to loneliness became baked into our nervous system.

In the short term, the stress of loneliness serves as a natural signal that nudges us to seek out social connection—just as hunger and thirst remind us to eat and drink. But when loneliness lasts for a long time, it can become harmful by placing us in a state of chronic stress. Researchers have found that chronic loneliness is associated with a greater risk of heart disease, dementia, depression, and anxiety. It’s also associated with a shorter life span. Being lonely raises mortality more than obesity or sedentary living does. The mortality impact is similar to that of smoking 15 cigarettes a day.

Chronic loneliness has many causes—greater geographic mobility; the often isolating effects of technology; a culture that defines success as the pursuit of power, wealth, and fame. When children are asked what their parents want most for them, they say their parents value achievement over being kind to others. But the less we prioritize connections with other people and the more we allow the quality of our interactions to decline, the more our social muscle begins to atrophy. Just like any other muscle, it weakens out of disuse. This can make high-quality social connection even harder.

That’s why the potential effects of physical distance from other people are so worrisome.