When Tobias van Schneider slips on a virtual reality headset to play Google’s Tilt Brush, he becomes a god. His fingertips become a fiery paintbrush in the sky. A flick of the wrist rotates the clouds. He can jump effortlessly from one world that he created to another.

When the headset comes off, though, it’s back to a dreary reality. And lately van Schneider has been noticing some unsettling lingering effects. “What stays is a strange feeling of sadness and disappointment when participating in the real world, usually on the same day,” he wrote on the blogging platform Medium last month. “The sky seems less colorful and it just feels like I’m missing the ‘magic’ (for the lack of a better word). … I feel deeply disturbed and often end up just sitting there, staring at a wall.”

Van Schneider dubs the feeling “post-VR sadness.” It’s less a feeling of depression, he writes, and more a sense of detachment. And while he didn’t realize it when he published the post, he’s not the only one who has experienced this. Between virtual reality subreddits and Oculus Rift online forums, there are dozens of stories like his. The ailments range from feeling temporarily fuzzy, light-headed, and in a dream-like state, to more severe detachment that lasts days—or weeks. Many cases have bubbled up in the last year, likely as a response to consumer VR headsets becoming more widely available. But some of the stories date as far back as 2013, when an initial version of the Oculus Rift was released for software developers.

“[W]hile standing and in the middle of a sentence, I had an incredibly strange, weird moment of comparing real life to the VR,” wrote the video-game developer Lee Vermeulen after he tried Valve’s SteamVR system back in 2014. He was mid-conversation with a coworker when he started to feel off, and the experience sounds almost metaphysical. “I understood that the demo was over, but it was [as] if a lower level part of my mind couldn’t exactly be sure. It gave me a very weird existential dread of my entire situation, and the only way I could get rid of that feeling was to walk around or touch things around me.”

It seems that VR is making people ill in a way no one predicted. And as hard as it is to articulate the effects, it may prove even harder to identify its cause.

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The notion of virtual-reality devices having a physical effect their users is certainly familiar. Virtual-reality sickness, also known as cybersickness, is a well-documented type of motion sickness that some people feel during or after VR play, with symptoms that include dizziness, nausea, and imbalance. It’s so common that researchers say it’s one of the biggest hurdles to mass adoption of VR, and companies like Microsoft are already working rapidly to find ways to fix it.

Some VR users on Reddit have pointed out that VR sickness begins to fade with time and experience in a headset. Once they grew their “VR legs,” they wrote, they experienced less illness. Van Schneider has noticed the same thing. “[The physical symptoms] usually fade within the first 1–2 hours and get better over time,” he wrote. “It’s almost like a little hangover, depending on the intensity of your VR experience.” Indeed, VR sickness is often referred to as a “VR hangover.”

“I was very fatigued. I was dizzy. And it definitely hits that strange point where the real world feels surreal.”

The dissociative effects that van Schneider and others have written about, however, are much worse. In an attempt to collectively self-diagnose, many of the internet-forum users have point ed to a study by the clinical psychology researcher Frederick Aardema from 2006 — the only study that looks explicitly at virtual reality and clinical dissociation, a state of detachment from one’s self or reality. Using a questionnaire to measure participants’ levels of dissociation before and after exposure to VR, Aardema found that VR increases dissociative experiences and lessens people’s sense of presence in actual reality. He also found that the greater the individual’s pre-existing tendency for dissociation and immersion, the greater the dissociative effects of VR.

Dissociation itself isn’t necessarily an illness, Aardema said. It works like a spectrum: On the benign side of the spectrum, there is fantasizing and daydreaming — a coping mechanism for boredom or conflict. On the other side, however, there are more pathological types of dissociation, which include disorders like derealization-depersonalization (DPDR).

While derealization is the feeling that the world isn’t real, depersonalization is the feeling that one’s self isn’t real. People who’ve experienced depersonalization say that it feels like they’re outside of their bodies, watching themselves. Derealization makes a person’s surroundings feel strange and dream-like, in an unsettling way, despite how familiar they may be.

When I spoke with Aardema on the phone, he had been wondering why his paper from ten years ago had suddenly been getting so many hits on the science-networking site ResearchGate. His study measured mild dissociative effects — think, “I see things around me differently than usual” — so he emphasized that there is a need to explore how these effects may relate to mood and depressive feelings. “There was some indication in our initial study that feelings of depression were important in relation to dissociation,” he said.

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I’ve never felt depersonalization, but I have felt derealization, the result of a severe panic disorder I developed when I was 25. It was nothing short of nightmarish. When the effects were tolerable, it felt like I was permanently high on psychedelics — a bad trip that wouldn’t end. When it was at it’s most intense, it was like living in my own scary movie: You look around at your everyday life and nothing feels real. Even faces that I knew and loved looked like a jumbled mess of features.

DPDR often occurs after a traumatic event, as a defense mechanism that separates someone from emotional issues that are too difficult to process. My case was triggered by stress. But according to a 2015 study in the journal Multisensory Research, feelings of unreality can also be triggered by contradicting sensory input — like one might experience inside a VR headset.

The study, by Kathrine Jáuregui-Renaud, a health researcher at the Mexican Institute of Social security, explains that in order for the mind to produce a coherent representation of the outside world, it relies on integrating sensory input—combining and making sense of the information coming in through the senses. When there’s a mismatch between the signals from the vestibular system — a series of fluid-filled tubes in the inner ear that senses balance — and the visual system, the brain short-circuits. Part of the brain may think the body is moving, for instance, while another part thinks the feet are firmly planted on the ground. Something feels amiss, which can cause anxiety and panic.

VR’s very purpose is to make it difficult to distinguish simulation from reality.

This, Aardema pointed out, could explain why books, movies, and video games don’t tend to cause the same kinds of dissociative aftereffects. Books don’t have moving visuals, and the movement in movies and video games is usually not intense enough. It also helps that these experiences are usually enjoyed while sitting still. So they just don’t have the same capacity to offset balance and vestibular function. (Though for some, movies can cause motion sickness. And for those people there is Moviehurl.com — a website devoted to rating movies on their likelihood of giving a viewer motion sickness.)

Scientists also believe that this kind of conflicting information is what causes motion-sickness symptoms like nausea and dizziness. So why do some VR users get motion sickness, while others end up experiencing something more serious? Research suggests that there is a link between serotonin levels, which play a role in mood regulation, and the vestibular system. So for those that may already suffer from a serotonin-related imbalance, like the 40 million Americans who suffer from anxiety disorders, VR’s disruption of the vestibular system may have a more profound effect.

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As van Schneider illustrated in his blog post, the appeal of virtual reality’s “superpowers” are compelling. VR’s very purpose is to make it difficult to distinguish simulation from reality. But what happens when the primitive brain is not equipped to process this? To what extent is VR causing users to question the nature of their own reality? And how easily are people able to tackle this line of questioning without losing their grip?

One evening during my DPDR phase, I was riding in a cab down a main street in the West Village, looking out the window. It was summer and there were tourists everywhere, and the light before sunset was still lingering. It was a perfect time to be out in the street, walking with friends and family, taking in New York City. But I remember the distinct feeling of hating everyone I saw. They had brains that just worked, brains that collected streams of sensory information and painted a satisfying picture of reality, just like brains are supposed to do. They most likely never questioned if what they were experiencing was real.

For some people, at least, it seems that VR could change that. In March, Alanah Pearce, an Australian video game journalist and podcast host, recounted troubling post-VR symptoms after the Game Developers Conference in San Francisco. “I was very fatigued. I was dizzy. And it definitely hits that strange point where the real world feels surreal,” she said. “I’m not going to go into that too in-depth, because it’s something I haven’t yet grasped. But I know that I’m not alone, and other people who play VR feel the same thing, where it’s like, nothing really feels real anymore. It’s very odd.”