(CNN) Rising rates of loneliness may not be news, but the three periods when it peaks may come as a surprise: More people reported feeling moderate to severe loneliness during their late 20s, their mid-50s and their late 80s than in other life periods, according to research published Tuesday in the journal International Psychogeriatrics.

The general sense of isolation was also more prevalent than the researchers expected. A full three-quarters of all study participants reported moderate to high levels of loneliness, said Dr. Dilip Jeste, senior author of the study and a professor of psychiatry and neurosciences at the University of California, San Diego.

"One thing to remember is that loneliness is subjective. Loneliness does not mean being alone; loneliness does not mean not having friends," said Jeste, who is also director of UC San Diego's Center for Healthy Aging. "Loneliness is defined as 'subjective distress.' " It is the discrepancy between the social relationships you want and the social relationships you have, he said.

Within the dark clouds, Jeste also found a silver lining: An inverse relationship exists between loneliness and wisdom. "In other words, people who have high levels of wisdom didn't feel lonely, and vice versa," he said.

The friends we lack

Dr. Vivek Murthy, former US surgeon general, says the reduced life span linked to loneliness is similar to smoking 15 cigarettes a day, Jeste noted in the study's introduction. Meanwhile, some research suggests that loneliness is common in old age, and others say it is typical in young age. "They are somewhat split, so that's what we wanted to find out," Jeste said.

A total 340 San Diego County residents between the ages 27 and 101 participated in the study.

Jeste and his co-authors hypothesized that these community-dwelling participants, none with serious physical or psychological ailments, would report more loneliness in old age based on the "usual assumption that as people get older, they become more alone," he said. They were surprised when they discovered two peaks (during the late 20s and mid-50s) in addition to the one in the late 80s. The results do not explain the reasons why people feel lonely, but Jeste had his theories.

"So the late 20s is often a period of major decision-making, which is often stressful because you often end up feeling that your peers made better decisions than you did, and there's a lot of guilt about why you did this or did that," he explained. It's a period of stress, which increases loneliness, he said.

"The mid-50s is the midlife crisis period," Jeste said. Typically, your health begins to decline, and many people learn that they have pre-diabetes, say, or heart disease.

"You see some of your friends are dying, and really, it's the first time you realize that your life span is not forever," he added. "And the late 80s is, of course, a period when, if you're lucky to have survived to that age, then things start getting worse." Along with health issues, you may experience financial issues and the death of a spouse and friends, he said: "It's probably the most understandable of the three periods."

The surprising main finding of the study was the 76% prevalence of moderate to severe loneliness, Jeste said: "We thought that it would be little more than a third." Men and women felt equally lonely and to the same degree -- no sex differences were found in either prevalence or severity, he and his colleagues found.

The study also showed that loneliness is associated with declines in physical health, mental health and cognition, though this has been reported in the past.

The third main finding, the inverse relationship between loneliness and wisdom, was "surprising and interesting and actually positive -- an optimistic finding," according to Jeste. He and his colleagues measured the six components of wisdom in each participant: general knowledge of life; emotion management; empathy, compassion, altruism and a sense of fairness; insight; acceptance of divergent values; and decisiveness -- the ability to make quick, effective decisions when necessary.

The traits we need to develop

This inverse association between loneliness and wisdom is "suggestive of the role of personality in the development and persistence of loneliness over time," said Anthony Ong , a professor of human development at Cornell University and a professor of gerontology in medicine at Weill Cornell Medicine.

The focus on wisdom as a protective factor is "novel, but more research is needed to clarify the mechanisms underlying the reported association between wisdom and loneliness," Ong, who was not involved in the new research, wrote in an email.

A "full understanding" of the phenomenon of loneliness is far from complete, Ong said. "Questions remain about whether the associations between loneliness and health reflect the effects of loneliness." Research has found that loneliness is a known risk factor for cognitive decline, cardiovascular disease, high blood pressure, disability and depression.

Ong added that more studies "addressing the direct, indirect, and moderated effects of social isolation and loneliness on health are urgently needed."

"More research is also needed to clarify the brain mechanisms underlying the association between loneliness and cognitive decline in old age and the extent to which such decline is reversible through intervention," he said. He believes that combating loneliness "may play an important role in improving well-being and prolonging life."

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Jeste agrees that more research is needed and that answering the question, "How do you reduce loneliness?" is the "main goal." With suicide opioid abuse and now loneliness all at "epidemic" levels, Jeste believes there is "increasing stress in general society over the last few decades."

"People need to realize that [loneliness] is a common problem. It is a serious problem," said Jeste, who suggested that the six component traits of wisdom might be cultivated. "Loneliness is sad; nobody disagrees with that," he said. "But it is a little bit more under our control than some people think."