In Moscow this past summer, a woman drifted-off to sleep after playing Pokemon Go on her smartphone. Later that night, she was awoken by a crushing pressure. She opened her eyes and reportedly saw that she was being assaulted by a real-life Pokemon character. Not a person in a Pokemon outfit, an actual Pokemon. Panicking, but unable to speak, she struggled with the creature while her boyfriend slumbered ignorantly beside her. Eventually, she was able to rise, and the Pokemon vanished. After a brief search of her home, the woman proceeded to report the assault the police.

News of the woman's police report was quickly, and somewhat gleefully, picked up by a variety of international tabloids. It rattled about the internet, and eventually surfaced on my Twitter feed. But my first thought, as an experimental psychologist with a particular focus in anomalous perceptual experiences was, "Well, that could have happened to anybody." Although it's impossible to definitively explain this woman's experience, I nevertheless felt quite confident that this late-night Pokemon assault fit neatly into our existing understanding of sleep. Indeed, given what we now know about this mysterious neuropsychological state - and the strange sensations it can bring - one might arguably describe her experience as 'normal'.

Imagery from your dreams can actually intrude into your waking reality

The short, seemingly paradoxical, explanation is that she could have been awake and she could have been dreaming. Setting aside the Pokemon for a moment, let's first consider her report of waking up, unable to move, with a crushing presence on top of her. The technical term that might apply here is 'sleep paralysis,' a subtype of parasomnia, or sleep disturbance. Beyond the inability to move, these periods of wakeful paralysis are often accompanied with vivid multisensory hallucinations. Effectively, imagery from your dreams can actually intrude into your waking reality.

The content of hallucinations can often be thematically linked to the feeling of paralysis -  manifesting as visions of an intruder in the bed who is physically holding the sleeper down. Records of incidents attributable to sleep paralysis can actually be found throughout history and across cultures with records dating back at least as far as 400 BC. This first reference has been traced to the Zhou Li/Chun Guan, an ancient Chinese book on sleep and dreaming. The text includes a taxonomy of different types of dreaming and researchers have identified E-meng ('dreams of surprise') as sharing many of the characteristics that are now associated with sleep paralysis. Depending on the time period and cultural context, these nightmare visions can be interpreted in different ways.

Sleep paralysis researchers Brian Sharpless and Karl Dograhmji have collected 118 different terms from around the world that describe sleep paralysis-like experiences: Germans have terms for hexendrücken - witch pressing - and alpdrücken -  elf pressing. Norwegian folktales include svartalfar - evil elves that shoot people with paralysing arrows before perching on their chests. The Japanese have a term, kanashibari, in reference to being magically bound by invisible metal. In parts of Switzerland people speak of tchutch-muton, an evil nightmare fairy that disguises itself as a black sheep. Kurds refer to mottaka, an evil spirit that suffocates people in the night. The Iranians have a term called bakhtak, which refers to a type of jinn that sits on the sleeper's chest. Scientists have theorised that sleep paralysis experiences might be result in some modern accounts of alien abductions. So I don't feel it's a huge logical leap to include Pokemon assaults.  

If it weighs as much as a duck...

For comparison's sake, consider this account by Jon Loudner, who gave 'evidence' during the infamous Salem Witch Trials in 1692:

"... I going well to bed, about the dead of the night felt a great weight upon my breast, and awakening, looked, and it being bright moonlight, did clearly see Bridget Bishop, or her likeness, sitting upon my stomach. And putting my arms off of the bed to free myself from that great oppression, she presently laid hold of my throat and almost choked me. And I had no strength or power in my hands to resist or help myself. And in this condition she held me to almost day."

After Jon's testimony, Bridget was executed by hanging, making her the first of casualty of the Salem witch craze

Like the Muscovite woman in 2016, Jon experienced a vision of a figure on top of him, accompanied by a crushing sensation and paralysis, although in his case, the best explanation he could come up with was an assault by a local witch. You can see the distinct parallels with the Pokemon case that have emerged as hallmarks of sleep paralysis cases. He woke in the night, was unable to move, and had the experience of a figure on top of him, disrupting his breathing. As a rather dark historical aside, after Jon's testimony, Bridget was executed by hanging, making her the first of casualty of the Salem witch craze. Jon's evidence was not the entire basis for her conviction, and was 'corroborated' by the damning physical evidence that Bridget may have possessed an extra supernatural nipple (after being identified once, it vanished on a subsequent search).

Witchcraft is a less popular explanation for contemporary sufferers, but even today, the precise physiological mechanisms that result in sleep paralysis are still not entirely understood. What is known is that, typically, when we dream, our actions are confined to our imagination. We all have a built-in safety mechanism, which you can think of as something like a circuit breaker; it effectively blocks your brain's motor planning signals from becoming motor action signals. This mechanism prevents us from physically acting out the actions that we dream of making. Thus, when you're being chased by a monster in a dream, you don't actually rise up and charge into the bedroom wall, or evolutionarily-speaking, tumble out of your tree. However, our brains are highly complex systems, and, as such, are prone to the occasional glitch.

One such glitch is fairly well known: sleepwalking occurs when the paralysis eases too early, while you're still asleep. On the flip side, sometimes the paralysis lingers - even after you've awoken. This typically happens just on the threshold of sleep - either just as you're waking up or just as you're drifting off. You can be conscious, with your eyes open, but be completely unable to move your body. Again, this is a fairly common occurrence, but the experience can be understandably alarming.  

Such problems may be a consequence of more general sleep disruption. Researchers have shown that sleep paralysis experiences can be induced in laboratory participants when they are repeatedly woken from deep sleep. And outside of laboratories, it's not particularly unusual for people to experience sleep paralysis in their nightly lives. If you've never had an episode yourself, odds are that you know someone who has. Experts estimate that up to 50% of the population will experience sleep paralysis at least once in their lifetime; some people report that their episodes are regular nightly occurrence.

Personally, I've had a couple of late-night visits from faceless shadow people, although they've always kept their hands to themselves

It's even possible that existing surveys may significantly underestimate the actual prevalence, given the perceived stigma of reporting hallucinations and concerns it might be a sign of a mental illness or drug abuse. In fact, healthy people can experience sleep paralysis and hallucinations in the absence of any complications arising from psychiatric conditions or substance use. Understanding that the experience is relatively normal can go a long way towards assuaging some of the accompanying anxiety. Personally, I've had a couple of late-night visits from faceless shadow people, although they've always kept their hands to themselves.

Visions of Falling Blocks

But let's return to the strange case in Moscow. Why would this hallucination appear as a Pokemon of all creatures? Given their highly subjective nature, dreams are notoriously difficult to study scientifically. How do you empirically measure hallucinations that occur largely while people are unconscious and that they typically forget upon waking? That being said, it doesn't take an expert to identify a potential link between her Pokemon Go playing and the subsequent waking dream. In fact, the connection between video games and dreams is one of the better documented areas of research on the subjective experiences of dreamers. 

Sleeping might serve to 'consolidate' memories from our waking life 

In 2000, a team of scientists led by Robert Stickgold at Harvard Medical School reported that participants who played the video game Tetris would consistently report seeing game-related 'hypnogogic imagery'– they experienced visions of the iconic falling blocks just before falling asleep. Similar results have been obtained using other types of video games, such as a downhill ski arcade game, a virtual maze, and even Doom. This evidence has been used to support the idea that sleeping might serve to 'consolidate' memories from our waking life – consolidation is term that refers to the process of reinforcing and strengthening newly created memories. Various experiments have demonstrated that people who are given memory-based tasks will perform better if they're given the opportunity to sleep after learning. It seems as though after we've been engaged in a learning task, our minds might be using sleep as a sort of rehearsal space to practice problems.

Rat 'dreams'

More evidence comes from an examination of the brain activity in sleeping rats. Animals present scientists with an array of pros and cons in comparison to working with human participants. On one hand, inserting electrodes directly into rat heads requires considerably less paperwork. On the other hand, it's not practical to simply ask the rats what they're dreaming about, so scientists are left to infer precisely how the electrical activity might relate to phenomenological experience.

In one experiment, a team at MIT inserted electrodes directly into a part of the rats' brains known as the hippocampus. In both rats and humans, the hippocampus is the part of the brain, which among other functions, is strongly associated with the way we form memories of physical spaces. The electrodes allowed the researchers to observe the real-time neural activity of specific cells in the hippocampus - with a spike being recorded every time one of the cells was activated. While the rats were wired-up, they learned to navigate a physical maze in exchange for a food reward. Because the hippocampus is involved with spatial learning, the patterns of electrical activity in the hippocampus could be associated with the rat's location in specific parts of the maze.

But here's where this method becomes relevant to our Pokemon/Tetris dream discussion: after the rats had learned the maze, the scientists left the electrodes recording as the rats drifted off to sleep. As the rats slept, the cells in the hippocampus would light up with activity. And not just any activity - the patterns of activations that occurred while the rats slept corresponded with the pattern associated with the correct maze runs. Again, we can't ask the rats what they were actually experiencing, but the results suggest that the rats may have been running the maze in their dreams, effectively practicing the best possible runs that they'd learned before falling asleep. One caveat is that none of this work proves a direct causal link between dreaming and memories: dreaming itself might not cause the memories to be reinforced, but could simply be a kind of side-effect of the consolidation process. 

In other words, the Moscow woman's terrifying hallucination is not only a relatively 'normal' occurrence; it may even provide an interesting window into the nature of sleep and why people dream. And that's worth considering if you ever find yourself experiencing a fantastic vision when you're on the threshold of sleep. Remember: Don't panic (and don't necessarily go blaming people with extra nipples).

Matthew Tompkins is an experimental psychologist at the University of Oxford. He tweets as @MattLTompkins.

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