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.
One of the founders of a new discipline called social neuroscience, Cacioppo used functional Magnetic Resonance Imaging (fMRI) brain scans and advanced scientific techniques to document the roles of loneliness and social connection as central regulatory mechanisms in human physiology and behavior.
The authors traced the need for connection to its evolutionary roots. In order to survive, humans needed to bond to rear their children. In order to flourish, they needed to extend their altruistic and cooperative impulses beyond narrow self-interest and immediate kin. But in the environment of evolutionary adaptation, the only real safety was in numbers.
Just as physical pain is a prompt to change behavior (such as moving a finger away from the fire), loneliness evolved as a prompt to action, signaling an ancestral need to repair the social bonds. Feelings of loneliness take a variety of forms, Cacioppo said.
"There are three core dimensions to feeling lonely-intimate isolation, which comes from not having anyone in your life you feel affirms who you are; relational isolation, which comes from not having face-to-face contacts that are rewarding; and collective isolation, which comes from not feeling that you're part of a group or collective beyond individual existence," he said.
It is not solitude or physical isolation itself, but rather the subjective sense of isolation that Cacioppo's work shows to be so profoundly disruptive. Yet, outward circumstances such as moving to a new community or losing an intimate partner can trigger loneliness.