Most diseases are dangerous primarily to the person who has them. Some few, though, are dangerous to everybody else in the vicinity. This is doubly true if the sufferer can warp the fabric of reality with a stray thought.

Most diseases are dangerous primarily to the person who has them. Some few, though, are dangerous to everybody else in the vicinity. This is doubly true if the sufferer can warp the fabric of reality with a stray thought.

Everybody dreams. Most people tend to remember little to nothing of their dreams, but they still have them. Dreams happen only during a particular "phase" of a person's sleep cycle known as rapid eye movement (or REM) sleep, a phase that most people will enter for short periods several times per night. REM's name comes from the fact that while in that phase, a person's eyes can be seen to be moving rapidly behind their eyelids, and this seems to reflect the fact that in our dreams, we're active and moving around. The rest of our body doesn't move during REM because structures in the brainstem paralyze the body, so that even if part of the brain thinks it's moving your arms and legs, that signal doesn't make it to the limbs and get you into trouble. In a small percentage of people, however, this paralysis doesn't work correctly. In this case, the body will perform some of the movements from the dreams; the person may show unusual twitching, because their body is only incompletely paralyzed, or more rarely, the person might actually perform recognizable actions, albeit in a rather perplexed and lost manner. The actions movements might be as innocuous as stumbling out of bed, or might be rolling over and beating the heck out of the person lying next to you. This condition is known as either REM behavior disorder or REM sleep behavior disorder depending on what source you read.

Although it's uncommon, REM behavior disorder can result in significant harm being done to the sufferer or to bed-mates. It's uncommon for a few reasons, the foremost of which is that even when the body is incompletely paralyzed, whatever movements come through tend to be disorganized, shaky, and uncoordinated. Equally importantly, however, severe harm is uncommon because most people aren't really equipped to cause a great deal of harm bare-handed. This is doubly true given that most people go to bed unarmed and don't keep a weapon under their pillow, and furthermore, the disorder typically appears only after the age of 60, when most people are no longer in peak street-fighting shape. Consider, however, the adventurer. The adventurer is not your average harmless 60 year-old. The adventurer usually is in prime fighting form, even at older ages; based on the SRD rules, and as all those taverns owned by retired fighters show, most adventurers just keep getting more and more dangerous as they age, as opposed to real people, who usually get much less dangerous after middle age. In fact, the elder adventurer is usually far more deadly than the younger one, as there are relatively few sixty year olds who are still below level 10. The adventurer often does sleep with a weapon under his or her pillow, and it may be a good deal more dangerous a weapon than a simple loaded handgun. And lastly, the adventurer may not be limited by the harm they can cause with their fists. A fifteenth level fighter with a few feats or class features might be fully capable of punching through a wall and into the next room. This pales in comparison to the fifteenth level sorcerer, who might be able to reduce an entire inn to ashes without ever waking up.

REM behavior disorder shouldn't be a common thing in a fantasy world. Like any disorder that only appears later in life, many people in a fantasy world never live long enough to develop it. Of those that do, logically, only a tiny fraction of them would actually be adventurers. On the other hand, one adventurer who does develop the disorder could potentially cause an impressive amount of havoc without ever even realizing they were the cause.

In modern times, of course, we can control REM behavior disorder fairly easily, with inexpensive and very tolerable medications, but these medications probably aren't available in a fantasy setting. Going to bed drunk would actually be a very effective treatment for the disorder, because alcohol reduces REM sleep, but that leads to its own problems, not the least of which is a powerful character walking around with liver problems, chronically sleep-deprived and irritable. As far as magic, REM behavior disorder might be easy to cure, or very hard. Like Parkinson's disease, a disorder which evidence suggests may develop out of REM behavior disorder in a large number of people, the disorder is caused by chronic progressive brain changes. It isn't the result of an infection or an injury as much as it is brain changes related to aging and the slow brain changes that come with it, so depending on the storyteller, Remove disease or even Restoration might not be enough to cure it. A storyteller who wanted to turn the disorder into the focus of a quest might rule that nothing short of a Wish or Miracle can cure it, and then require a group of player characters to travel in search of such magic alongside an aged companion who poses the very real risk of blowing them up every time he or she goes to sleep. It might be the only time in your campaign that every PC volunteers to stand watch over night. 

More than four years ago, Dr. Eris Lis, M.D., began writing a series of brilliant and informative posts on RPGs through the eyes of a medical professional, and this is the one that appeared here on July 12, 2014. Lis is a physician, gamer, and author of the Skirmisher Publishing LLC OGL sourcebook Insults & Injuries, which is also available for the Pathfinder RPG system.