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Man standing alone

At any age, loneliness can have detrimental effects on overall health. It's a major risk factor for depression and various forms of anxiety as well as cognitive deficits. "Loneliness can change the neurochemistry of the brain, turning off the dopamine neurons, which trigger the reward response, and causing some degeneration in the brain when the reward response is not activated," says Katherine Peters, MD, PhD, FAAN, associate professor of neurology and neurosurgery at Duke University. According to a 2015 report in International Psychogeriatrics, loneliness is strongly correlated with lower cognitive function.

These effects may stem partly from the fact that "loneliness induces a chronic low-level inflammatory response where levels of cortisol and other stress hormones may be higher," says Louise McCullough, MD, PhD, chair of neurology at the McGovern Medical School at the University of Texas and chief of neurology at Memorial Hermann Hospital-TMC in Houston. "Loneliness is also associated with reductions in brain-derived neurotrophic factor, a protein crucial for neuronal health, cognition, and memory," she says."In my research, we have found an association between loneliness and performance on cognitive tasks," says Martina Luchetti, PhD, assistant professor of behavioral sciences and social medicine at the College of Medicine at Florida State University. Her studies have found that loneliness increases older adults' risk of developing cognitive impairment. And in those with mild cognitive impairment, she adds, "loneliness can predict the transition from mild cognitive impairment to dementia."

A study in JAMA Psychiatry in 2016 found that older adults who have normal cognitive function but who are lonely showed more amyloid in their brains on PET scans than their nonlonely peers. Research suggests that lonely people have a higher risk of dementia years later than those who are not lonely, says Dilip Jeste, MD, director of the Center for Healthy Aging at the University of California San Diego.

People who have a stroke and become socially isolated and lonely have a worse prognosis, including a 30 percent higher risk of a second stroke, Dr. McCullough says. "It may be due to poor compliance with their medications, but also to increases in inflammatory responses."

Meanwhile, a growing body of research suggests that loneliness can negatively affect hormonal, immunological, cardiovascular, and inflammatory responses. Loneliness has been implicated in everything from increased risk of hypertension and heart disease to a reduced antibody response to the flu vaccine. "If you're lonely, you can get sick more easily and remain sick for longer," says Ami Rokach, PhD, a clinical psychologist at York University in Toronto. "Loneliness is a form of stress that depresses the immune system."

In addition, loneliness is associated with an increased risk of premature mortality, according to a 2015 review in Perspectives on Psychological Science. "Chronic loneliness is linked with inflammation, which may explain some of the diverse outcomes in terms of physical, mental, and emotional health," says study co-author Julianne Holt-Lunstad, PhD, professor of psychology and neuroscience at Brigham Young University in Provo, UT.

Healthy lonely people are also more susceptible to accelerated cellular aging, according to a study in a 2019 issue of the Annals of Behavioral Medicine. And loneliness is associated with higher levels of sleep fragmentation. This is probably because humans need to feel safe in order to sleep well, says Louise Hawkley, PhD, senior research scientist with the research organization NORC at the University of Chicago. "Lonely people are susceptible to implicit feelings of anxiety that can impair their sleep quality."