The fear of being an empty-nester is something many parents experience as their children age. While this phenomenon may seem to be a social construct, studies show that loneliness can actually have a huge impact on your life expectancy.

The study: In a study conducted at the University of California, San Francisco, researchers found that loneliness can have a large impact on older adults and is associated with serious health problems and even death.

“In our typical medical model, we don’t think of subjective feelings as affecting health,” said Carla Perissinotto, assistant professor in the UCSF Division of Geriatrics, in a statement on the study. “It’s intriguing to find that loneliness is independently associated with an increased rate of death and functional decline.”

Researchers also found that, while those experiencing loneliness can experience symptoms of depression, the two are not mutually exclusive.

While depression is associated with a lack of employment, energy and motivation, loneliness can be felt by people who are fully functional but feel empty or desolate.

“Loneliness and weak social connections are associated with a reduction in lifespan similar to that caused by smoking 15 cigarettes a day and even greater than that associated with obesity,” former Surgeon General of the United States, wrote in an article for the Harvard Business Review, Dr. Vivek H. Murthy,

Why does it happen?: The reason loneliness tends to have such an impact on health is we enter “fight or flight” mode when we are separated from others, according to Murthy.

This triggers an increase in stress hormones throughout the body, creating physiological changes in the body. Repeatedly activating this stress response can eventually take a toll on the body, and issues can especially arise when the body is in that state long-term.

Because older people are more susceptible to immune challenges, as well as infections and diseases, social isolation and loneliness could further compromise their overall health, according to the research.

Since the body can also only handle so much stress, if stress beats down the immune system early on in life, it can take a significant toll later on, according to Murthy.

“Loneliness is taking a heavy toll on our nation’s seniors,” said Tamara Lynn Meadows, divisional director of clinical operations in Oklahoma at StoneGate Senior Living. “As research continues to quantify the realities of senior isolation, healthcare professionals are focused more than ever before on spotting lonely patients and determining the best way to support them.”

What loneliness really does to you: In research conducted on social isolation, John T. Cacioppo, director of the Center for Cognitive and Social Neuroscience at the University of Chicago, suggested that loneliness results in increased sympathetic tone, decreased inflammatory control and decreased sleep. Other studies have also found a correlation between cardiovascular disease and depression and loneliness.

Death: According to StoneGate, loneliness increases the likelihood of mortality by 26%. The research found lonely individuals have a 64% higher chance of developing clinical dementia, too. Coronary bypass patients who report feeling lonely have a mortality rate five times higher than other patients 30 days post-surgery.