The first time I thought about heaven, it terrified me.

I was 4 years old, and my grandfather had just passed away after a drawn-out battle with cancer. My family tried to console me by explaining that he went to a beautiful, calm, and happy place, where he would be united with all his loved ones, forever and ever.

That night, I lay in bed in pitch darkness and tried to grasp what they had told me. But every time I thought I had a grip on eternity, it slipped further away. The largest number of years I could imagine failed to make a dent in infinity. My primitive brain filled with an existential angst. The idea of living forever was even more unsettling than the idea of no longer existing after death.

Woody Allen once said, “Eternity is a very long time, especially toward the end!” Eternity sounds great on the surface, but actually experiencing it may be an entirely different matter. For some people, the very notion of infinity sends chills up the spine. In fact, for many who suffer from “apeirophobia”—a term for the fear of eternity—the thought of an existence that goes on forever amounts to torture.

“I remember the first time it hit me,” writes Kellie, the username of an anonymous commenter on the pop culture website YOMYOMF, which featured a brief story about an actor’s experience with apeirophobia . “I was around 8 years old. Now I’m in my 30s, and the thought of eternity still freaks me out. It usually hits at night when I’m trying to sleep. I’ve learned to push it out of my mind, but sometimes I can’t, and when that happens I start pacing the room and thinking that I might have to go to the emergency room or else I might kill myself.”

Search for apeirophobia on Google and you’ll find hundreds, if not thousands, of message-board posts and comments similar to this on popular Internet forums like Reddit, Quora, and Yahoo! Answers. Websites dedicated to phobia help and support, such as Phobia Fear Release and MedHelp, all have comment threads on apeirophobia, too. On these sites, Apeirophobes share experiences and swap advice. Even though most describe dealing with unrelenting anxiety, many seem comforted by knowing they aren’t alone.

Despite all this discussion, there is so little research on apeirophobia that it lacks its own Wikipedia entry. It is not explicitly recognized by the Diagnostic and Statistical Manual of Mental Disorders, the American Psychiatric Association’s standard reference for psychiatry, but it does meet their criteria for a “specific phobia,” which is classified as an anxiety disorder. It is included on many sites with informal phobia lists, but it is absent from the websites of the Mayo Clinic and the National Institute of Mental Health, as well as WebMD. A Google Scholar search doesn’t yield a single quantitative study on the phobia.

Grasping for some sense of its prevalence, I posted on Facebook, asking if anyone had experience with apeirophobia. I received considerably more responses than expected. One friend said that he used to get so overwhelmed during sermons in early adolescence that he felt physically sick. Another wrote, “The thought of eternity still fills me with anxiety when I wake up from naps. My solution: I stopped napping.”

Where does this fear come from? A realization that an eternal afterlife could become infinitely repetitive? The recognition that one lacks control over their own destiny?

Martin Wiener, a former colleague of mine at George Mason University who researches the neural underpinnings of time perception, notes that the brain region hypothesized to control long-term planning, the frontal lobe, is one of the last to develop in humans as they grow. “In adolescence, there is a dawning realization that occurs where one realizes they will become an adult,” Wiener says. “I suspect that, in apeirophobia, one comes to the ‘realization’ that after death you will live forever (if you believe this), and in simulating that experience in your mind, one realizes that there is no way to project ahead to ‘forever’—and that experience is, inherently, anxiety-provoking. As such, the anxiety that these folks are feeling may not be much different than the fear of growing up, getting old, or death.”

Maybe human brains, as finite instruments with limited cognitive and computational capacities, are flat-out not hardwired to have a conception of something completely absent from sensory experience. Evolution has done just fine without organisms that contemplate infinity, after all. Doing so wouldn’t have likely offered any survival advantages to pre-modern humans.

Wiener’s explanation would predict that apeirophobia creeps up in younger stages of life, and although we cannot know anything for sure without sufficient data, the reports scattered across the web seem to roughly match this concept. But there does seem to be a small percentage of people with apeirophobia who first experienced fear even before adolescence, such as myself. For that, Wiener offers a different—but likely compatible—idea regarding the phenomenon: What if a fear of the afterlife is simply a religious manifestation of a fear of death that’s innate in everyone?

“Terror Management Theory, which is derived from Ernest Becker’s 1973 Pulitzer Prize-winning work of philosophy and psychology The Denial of Death, essentially says people in modern civilization are all walking around in denial of our mortality—and that culture, religion, and entertainment all exist for the purpose of distraction,” Wiener explains. “The idea is that these things keep us from thinking about our own deaths, and that systematically, people are repressing this fear. The fear of eternity could just be that same fear manifesting itself in a different way.”

There shouldn’t be too many atheists who fear of eternity, since they reject the idea of an afterlife, Wiener says. But that doesn’t mean that those who aren’t religious are immune to existential anxiety. Infinity, after all, doesn’t pertain only to time; it can also apply to space. “I feel that we are all insignificant compared to the universe,” wrote Jamie Adkins, a nurse and longtime friend of mine, in response to my Facebook post. “When I start to think beyond our solar system, it is as if my thoughts automatically stop to protect myself from having some form of a panic attack. The knowledge of black holes will give me nightmares for days. The thought of the distance between galaxies is unbearable.”

She likened the experience to Horton Hears a Who. “We are on this tiny flower and can be blown away any second.”

It’s an age-old anxiety. In the 17th century, the French mathematician and philosopher Blaise Pascal described in Pensées, how both infinite space and time shook him to the core: “When I consider the short duration of my life, swallowed up in the eternity before and after, the little space which I fill, and even can see, engulfed in the infinite immensity of spaces of which I am ignorant, and which know me not, I am frightened, and am astonished at being here rather than there; for there is no reason why here rather than there, why now rather than then.”

Bilbo Baggins, burdened with unusually long life thanks to the almighty ring in Tolkien’s The Lord of the Rings, put it this way: “I feel thin, sort of stretched, like butter scraped over too much bread.”

Although some apeirophobics have benefitted from treatments like medication and cognitive behavioral therapy, others have found no way to mitigate the existential terror. Personally, I’m no better at grasping infinity than I was at 4. But rather than trying to comprehend eternity, now I just avoid the thought altogether.