Feeling lonely won't kill you. Actually being alone might.

A new study published in the Proceedings of the National Academy of Sciences finds that after adjusting for demographic factors and underlying health, self-reported feelings of lonesomeness have no significant connection to mortality among the elderly, but actual social isolation increases the likelihood of death by a stunning 26 percent.

Public-health researchers have long known there's a connection between loneliness, social isolation and mortality. What they didn't know was whether loneliness was the emotional mechanism through which actually being alone affected health, or whether the feelings of loneliness and the effects of social isolation were somehow independent.

Consider the case of the hermit. "We all know people who might seem social isolated but don’t experience loneliness because they are comfortable being on their own," says Andrew Steptoe, one of the study's authors. Would they be at higher risk of death as well?

Steptoe and his coauthors, Aparna Shankar, Panayotes Demakakos and Jane Wardle, decided to find out. The researchers looked at 6,500 men and women who participated in the English Longitudinal Study of Aging (ELSA) in 2004 and 2005. They measured the participants on feelings of loneliness and objective measures of social isolation. And then they looked at mortality rates over the next seven years.

At the outset, both social isolation and self-reported lonesomeness increased the risk of death. But after controlling for other factors, the effects of lonesomeness dropped away. The effects of true isolation, however, persisted. Too much time without human contact can kill you.

But if the mechanism isn't lonesomeness, what is it? "Social isolation has practical as well as emotional aspects," says Steptoe. "People with few social contacts may not have people around them who can give them advice, recommend that they go to a doctor with symptoms, ensure that they maintain healthy lifestyles, or perhaps they don’t have anyone around when they experience acute symptoms."

Alleviating social isolation can be easier than we think. A phone call, perhaps even an e-mail, counts. "The take home message is that we need to keep an eye on the social connections of older people, since maintaining social contacts among seniors and reducing isolation may be particularly important for their future survival," says Steptoe.

In other words, call your grandmother.