Jody Schoger felt utterly alone, "curled up like a turtle" in her hospital bed, where she was fighting a life-threatening infection after breast cancer surgery.
"I remember never even opening the blinds, just hibernating," says Schoger, of The Woodlands, Texas. "I even started sleeping with the blankets pulled over my head. I was at the edge of the world."
Like many people with serious illness, Schoger found herself cut off from family, friends and the "real" world outside the hospital, which began to feel like another planet. Although many people would have been happy to help, Schoger says she never thought to call them. And while the hospital was filled with doctors, nurses and other patients, Schoger -- facing her own mortality -- felt very much alone.
As her story suggests, the pain of loneliness is caused less by being alone than by feeling alone, says John Cacioppo, director of the University of Chicago's Center for Cognitive and Social Neuroscience.
Researchers are studying the causes and health effects of loneliness -- both on the body and mind -- in the hope of helping individuals and communities remain healthy and connected. Lonely people tend to have higher blood pressure and weaker immune systems, he says. Loneliness may even affect our genes. In lonely people, genes that promote inflammation are more active, while genes that reduce inflammation are less active, he says.
Considering how humans evolved, it makes sense that feeling alone gives people stress, Cacioppo says.
Humans evolved to depend on one another. Those who fail to connect with others are more likely to die without passing on their genes, Cacioppo says. In many ways, he says, the drive to avoid being alone is as strong as the need to alleviate hunger, thirst and pain.
The desire for companionship -- and the fear of being ostracized -- even motivates people to behave better, Cacioppo says.
"Loneliness is a biological process that contributes to being better social members of our species," he says. "Think about what happens when you give a toddler a timeout. You basically make them feel lonely. Then they come back and are more likely to share, to be generous, to take the perspective of the other."
Scientists don't really know the effects, however, of longer periods of enforced isolation, such as solitary confinement, says Cacioppo, who is interviewed in a new National Geographic Channel "Explorer" episode that premieres Sunday.
Former inmate Josue Gonzales says it was hard to readjust to being in crowds after his October release from the Colorado State Penitentiary. Gonzales served nearly a decade in prison, with about half of that in solitary, after stealing a car and breaking into a house.
"At first, when I got out, I didn't even want to go to the restaurant that my family wanted to take me to," says Gonzales, 29, who also appears in the "Explorer" episode. "There were so many people in there. Then I got in there and sat down, and I got nervous and started shaking."
But psychologist Peter Suedfeld, professor emeritus at the University of British Columbia in Vancouver, notes that interviews with veterans of solitary confinement can be misleading. That's partly because people sent to solitary confinement are likely to be very different from other people.
The only definitive experiment, Cacioppo says, would be to randomly assign half of prisoners selected for solitary confinement to return to the general prison while allowing the rest to proceed to solitary as planned. That would allow researchers to compare how the two groups change over time, if at all, and to determine whether those changes were the result of solitary confinement or something else.
Gonzales, who now lives in Pueblo, Colo., recently got married and is looking for work. He says he's trying to rebuild his social connections.
And Schoger -- who has been cancer-free for 12 years -- says she sometimes finds support in unexpected ways.
"If any survivor posts something onto Twitter or Facebook that they're 'having a hard day,' I can bet you 10 to 1 that he or she is surrounded by good wishes by day's end," she says.
"Yet the survivor, the one who is ill, has to be willing to take that step. Once he or she does, the burden of illness and its perceived isolation fades away."
Explore further: Loneliness associated with increased risk of Alzheimer's disease