Ask someone to recount a time they felt lonely, and they’ll undoubtedly have a story to share. You might hear about the college freshman away from home for the first time. Or the new mother feeding her baby in the dark stillness of 4 a.m.

“Most people feel lonely at some point during their lives,” writes researcher Ahmet Akin of Sakarya University. “As social animals who participate extensively in social relationships, humans open themselves up to the possibility of loneliness.”

Researchers find that loneliness ebbs and flows as we age, in relatively predictable ways. Counterintuitively, we tend to be lonelier when young—and also when old. Among those high-risk groups, as many as one quarter of people may feel lonely on a regular basis. Understanding why we become lonely in certain life stages can help us to handle the uneasy feelings of isolation when they inevitably arise.