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The effects of social isolation go well beyond emotional loneliness to what some researchers are now calling "a silent killer."

Like many similar studies, the current study in the journal SLEEP focused on fruit flies rather than human subjects, which may seem strange. But because of genetic similarities, fruit fly studies have yielded an astonishing number of insights into human social and biological realities.

Socially isolated fruit flies had less sleep, which led to the ignition of the unfolded protein response (UPR). This response protects cells from stress in the short-term, but when activated for a long-term period, the end result is cellular stress. What's more, there is a link between chronic activation of the UPR and diseases associated with aging, such as Alzheimer's.

This combination creates a perfect storm for elderly people.

"When animals get older, you start to see a more maladaptive UPR," said senior author Nirinjini Naidoo. "A lot of elderly people live alone, and so we suspect that stresses from the combination of aging and social isolation creates a double-whammy at the cellular and molecular level. If you have an age-related disruption of the UPR response, compounded by sleep disturbances, and then you add social isolation, that may be a very unhealthy cocktail."

The researchers at the University of Pennsylvania are continuing their study on the links between aging, sleep deficiency, cellular stress, and age-related diseases. Aging is a key ingredient to research in the context of social isolation.

"Aging itself seems to make the UPR more defective, but we suspect this is worsened by the fact that aging also tends to cause more fragmented sleep," said Naidoo.