Loneliness can be as harmful to your health as smoking and obesity, claim psychologists.
Lack of connection with others not only makes us unhappy but it is also bad for the wellbeing of the body and mind, research finds.
A sense of rejection or isolation increases blood pressure, stress levels and general wear and tear as well as increases your chances of developing Alzheimer's Disease.
It also reduces will power and perseverance, thus affecting the ability to follow a healthy lifestyle, according to scientists.
The findings were outlined by Professor John Cacioppo, of the University of Chicago, at the American Association for the Advancement of Science annual conference.
Loneliness not only alters behaviour, but loneliness is related to greater resistance to blood flow through your cardiovascular system, Professor Cacioppo said.
Loneliness leads to higher rises in morning levels of the stress hormone cortisol, affects the immune system, higher blood pressure and an increased level of depression.
Loneliness, or perceived social isolation, also is related to difficulty getting a deep sleep and a faster progression of Alzheimer's disease, said Professor Cacioppo.
Healthwise, he said the difference between a lonely person and a popular person was akin to "a smoker and a non-smoker".
"That stunned all of us, myself and all my colleagues in terms of the effects it had," he said. "It shows just how powerful it is.
"The lonely have poor health. They exercise less, are more likely to quit. Eat more calories. They comfort eat more fats and sugars.
"Loneliness lowers the ability to control yourself. It is really easy after a bad day to have a second scotch and a third to get some comfort."
One of the founders of a new discipline called social neuroscience, Professor Cacioppo, traced the need for connection to its evolutionary roots.
In order to survive in the past, humans needed to bond to rear their children. In order to flourish, they needed to extend their altruistic and cooperate, he concluded.
Just as physical pain is a prompt to change behaviour, such as moving a finger away from the fire, loneliness evolved as a prompt to action, signalling an ancestral need to repair the social bonds.
The problem of social isolation is likely to grow as conventional family structures die out, said Professor Cacioppo, the author of Loneliness: Human Nature and the Need for Social Connection.
People are living longer, having fewer children later in life and increasingly mobile around the world.
Surveys also show that people report significantly fewer close friends and confidants than those a generation ago.
By Richard Alleyne, Science Correspondent in Chicago