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Insomniacs know what it's like to spend hours by themselves. The experiences that come with the can't-sleep disease — fighting consciousness at 1am, staring into darkness at 2am, skulking around the house at 3am — amount to forced, late-night solitude. Many people, from Shakespeare to Heath Ledger to tweens with Tumblr accounts, have pointed out that sleeplessness is a lonely form of torment. Now, scientists are trying to make sense of the well-observed link between insomnia and loneliness: A new study, published in the journal Psychiatry Research, explores the possibility that insomnia increases one's risk for loneliness and suggests that this phenomenon is partially explained by depression. 

At any given moment, about one-third of adults are dealing with at least one symptom of insomnia, i.e., they can't fall or stay asleep, or don't feel refreshed from the sleep they're getting. And, in recent years, behavior scientists have put out a stream of research linking loneliness to chronic diseases, shortened life span and impaired immune function. The uptick in loneliness, experts have posited, may be caused both by the rise of virtual (rather than IRL) interactions and the fact that people are living longer than they used to.

The bulk of research connecting loneliness and insomnia depicts insomnia as a product of loneliness. Lonely people, studies suggest, go on to develop insomnia and other sleep problems. Let's call this the "Ledger Effect," as a nod to what Pinterest says is Heath Ledger's most famous quote: "I think the most common cause of insomnia is simple; it's loneliness." 

One way to explain the Ledger Effect is that lonely people lack social security, leaving them hyper-alert and unable to shut off their brains come bedtime. In one 2013 study, as Slate reported, participants "who were lonely were far more prone to micro awakenings, which suggest the brain is on alert for threats throughout the night, perhaps just as earlier humans would have needed to be when separated from their tribe." 

What about the other way around? The possibility that insomnia causes loneliness hasn't received as much attention as the Ledger Effect has. But it should. Staying awake all night long is in itself a lonely endeavor. Not to mention, insomnia affects how well people perform cognitive tasks, regulate their emotions and engage in social situations. An insomniac might not be in the right state of mind to banter with colleagues or even sit through dinner with friends. And friendless people get lonely. 

For the current study, a Florida State University-lead team of psychologists re-analyzed previous research linking sleep problems and friend problems to see if insomnia predicted subsequent loneliness. They also wanted to see if the insomnia-loneliness connection persisted after taking into account other psychiatric problems. In other words, does insomnia itself cause loneliness or do insomniacs get lonely due to underlying anxiety or depression?

Researchers re-analyzed six studies on different sub-populations: college students, military recruiters, adults with a history of suicidal thoughts, psychiatric outpatients and young adults at risk for suicide. And they found a consistent association between insomnia and loneliness. But four of the studies were cross-sectional, meaning they only provided data about one group of people at one point in time. A cross-sectional study can show that two characteristics (e.g., loneliness and insomnia) are statistically linked, but it can't explain how or why they're linked. 

The two studies that weren't cross-sectional, however, allowed researchers to see if insomniacs were more likely than non-insomniacs to become lonely. And that's what the studies showed, more or less. In the study on young adults, insomnia significantly predicted loneliness five weeks down the line. But the (inverse) Ledger Effect showed up, too. These findings lead researchers to hypothesize that insomnia and loneliness have a cyclical relationship: insomnia breeds loneliness, which in turn aggravates insomnia, and so on. 

The re-analysis also highlighted the potential role of depression in the insomnia-loneliness link. The studies uniformly showed that insomnia and loneliness emerged independently of problems including anxiety and nightmares — insomniacs tended to be lonely whether or not they were also plagued by panic or scary dreams. But researchers were not able to discount depression in the same way. In a few of the studies, participants proved unlikely to report insomnia and loneliness unless they were depressed as well. 

In some ways, it shouldn't be surprising that depression reared its head. As researchers pointed out, depression is heavily (and separately) associated with both insomnia and loneliness: People who are depressed often (if not most of the time) report sleep disturbances. And, while loneliness isn't considered a symptom of depression, it does seem to be a consequence of clinical-grade sads. On the other hand, insomnia and loneliness have been linked in non-depressed people. So it's possible that insomnia and loneliness affect each other differently depending on who's being studied. 

While the current study doesn't clarify the relationship between insomnia, loneliness and depression, it does support the link between insomnia and loneliness, set the stage for researchers to explore insomnia as a precursor to loneliness and help identify next steps. Going forward, researchers wrote, it would make sense to consider symptoms of depression and social isolation while experimenting with insomnia treatment methods.

For now, insomniacs should remember that somewhere between 6 percent and 15 percent of Americans share their struggle. They might not sleep next to people who are hopelessly wired at 4am. But new insomni-buds are just a weird Reddit-hole away.