https://www.sciencedaily.com/releases/2020/12/201215082059.htm
The researchers found several differences in the brains of lonely people. These brain manifestations were centred on what is called the default network: a set of brain regions involved in inner thoughts such as reminiscing, future planning, imagining and thinking about others. Researchers found the default networks of lonely people were more strongly wired together and surprisingly, their grey matter volume in regions of the default network was greater. Loneliness also correlated with differences in the fornix: a bundle of nerve fibres that carries signals from the hippocampus to the default network. In lonely people, the structure of this fibre tract was better preserved.