Researchers have found an unexpected link between strange, physically violent dreaming and the risk of neurological disease, such as Parkinson's and dementia, later in life, and say they could act as an early warning sign decades before diagnosis.
It's not yet clear how definitive this link is, but the results are supported by previous research that's given patients with violent, physical dreams an 80-100 percent chance of eventually developing a neurodegenerative disorder.
These rare dream disorders, known as REM sleep behaviour disorder (RBD), cause people to 'act out' their dreams - kicking and punching in their sleep, while sometimes shouting and screaming, and maybe even flying out of their bed in a violent rage.
"The consensus among all RBD researchers is that it's not a matter of if, but when," sleep expert Carlos Schenck from the Minnesota Regional Sleep Disorders Centre in Minneapolis, told Wired back in 2014.
"Basically, the longer you follow these [patients], the more they will convert to a neurodegenerative disorder."
REM, or rapid eye movement, sleep is one of three states we typically transition through each night, and it's the only one associated with dreaming.
During REM sleep, the electrical activity of the brain actually looks quite similar to how it would look during the day, except that while the neurons are firing like they would when you're awake, the body typically experiences temporary muscle paralysis.
While this temporary paralysis does allow for some muscle twitches and mumbling if you're a sleep-talker, most people remain fairly still through this stage of sleep (sleepwalking is associated with non-REM sleep).
But if you happen to develop REM sleep behaviour disorder, your dream stage will be very different - patients often perform physical actions that match their very vivid dreams.
It's not clear what causes an onset of RBD, but it can appear at any age - and oddly enough, older men are far more susceptible than women and children (the ratio is as high as 9:1 male to female).
Neuroscientist John Peever from the University of Toronto, Canada, has been investigating a link between RBD and neurological diseases that seems to keep cropping up in human studies.
By focussing on the brainstem, which for decades has been associated with dream formation, Peever was able to isolate a specific group of cells that appear to be responsible for maintaining the REM sleep.
When he identified this group of cells in mice, he was able to quickly transition the animals between REM and non-REM sleep by simply turning the cells on and off.
With this in mind, Peever and his team looked at how this group of cells were functioning in human RBD patients, and found that they were damaged - and that's important, because this damage appears to also be linked to the onset of neurological diseases.
"For some reason, the cells in the REM sleep area are the first to be sickened, and then the neurodegenerative disease spreads up into the brain and affects the other areas that cause disorders like Parkinson's disease," Peever told Live Science.
"REM Behaviour Disorder is in fact the best-known predictor of the onset of Parkinson's disease."
While Peever's results are only preliminary and not yet peer-reviewed, they are similar to those of previous studies - and unfortunately for RBD patients, they percentages are incredibly high.
In one 2013 study involving 44 RBD patients, researchers found that 82 percent of them had developed neurological disorders within 10 years of observation.
In another published that same year, of the 26 RBD patients studied, 80.8 percent ended up developing Parkinson's or dementia.
A study from 2010 found that 41 of their 43 RBD patients (95 percent) had developed a range of neurological disorders.
The majority had a progressive neurodegenerative disorder called multiple system atrophy (MSA), and a previous study in 2007 saw 100 percent of its MSA patients diagnosed with RBD.
Now, Peever says his study has not only supported these high percentages, but by identifying damage in the brain stem cell group as the trigger for neurological disorders, he's "taken away the correlation [to] show causality".
"We observed that more than 80 percent of people who suffer from REM sleep disorder eventually develop synucleinopathies [neurological disorders], such as Parkinson's Disease and Lewy bodies dementia," he says in a press statement.
"Our research suggests sleep disorders may be an early warning sign for diseases that may appear some 15 years later in life."
While more research is needed to figure out what exactly is going on here, it's at least given us one pretty solid indication of the kinds of bad dreams we should be worrying about.
For the rest of us who don't physically act out our dreams? The jury's still out on that.
The preliminary results of Peever's study were presented at the 2017 Canadian Neuroscience Meeting in Montreal this week.