While it is well known that feeling connected to others is vital to mental health, as well as physical health, being isolated from others can result in obesity or addiction to smoking.

The research, which has been reported in a new book, Loneliness: Human Nature and the Need for Social Connection, suggests that chronic loneliness belongs among health risk factors such as smoking, obesity or lack of exercise, according to lead author John Cacioppo, the Tiffany and Margaret Blake Distinguished Service Professor in Psychology at the University.

"Loneliness not only alters behavior, but loneliness is related to greater resistance to blood flow through your cardiovascular system," Cacioppo said.

"Loneliness leads to higher rises in morning levels of the stress hormone cortisol, altered gene expression in immune cells, poorer immune function, higher blood pressure and an increased level of depression," he added.

Loneliness also is related to difficulty getting a deep sleep and a faster progression of Alzheimer's disease, said Cacioppo. He drew on recent research in preparing the book, written with William Patrick, the former science editor at Harvard University Press.

The book has been published by W.W. Norton.