Every night around 9 p.m., my father started narrating his dreams out loud, sometimes punching the air, once even falling out of bed after what he swore was a fight with an intruder. “It’s just his age,” my mother said, waving it off as the kind of restless sleep that comes with getting older. A geriatric memory specialist we consulted three years later for something unrelated looked up the moment I mentioned it and said, almost too calmly: “How long has this been happening?”
What she was describing has a name: REM sleep behavior disorder, or RBD. And according to a growing body of neurological research, it isn’t just quirky sleep. It’s often one of the earliest visible signals of a brain disease that hasn’t shown up anywhere else yet, not in conversation, not in memory, not in daily function.
Key takeaways
- A sleep behavior most families dismiss as ‘just restless sleep’ is often the earliest detectable signal of serious neurological disease
- Research shows 90% of people with this specific sleep disorder will eventually develop Parkinson’s or dementia—but the window for early detection spans years
- Cognitive decline in these cases begins up to 10 years before diagnosis, starting with attention problems most people never recognize as warning signs
When dreams stop staying inside the body
Normally, the brain paralyzes most of the body’s muscles during REM sleep, the stage when dreams happen. That paralysis is a safety mechanism. In RBD, it fails. People physically act out their dreams: yelling, kicking, grabbing, sometimes yelling, thrashing, or acting out their dreams, sometimes violently enough to injure a bed partner. Families tend to file this under snoring, stress, or “he’s always been a restless sleeper.” A sleep specialist files it under something else entirely.
“It’s not just restless sleep, it’s a neurological warning sign,” says Shady Rahayel, a neuropsychologist and researcher at the Centre for Advanced Research in Sleep Medicine at Sacré-Cœur Hospital in Montreal. That framing matters, because RBD isn’t a side effect of aging brains getting a little noisier at night. It’s frequently the first crack in a much longer neurological timeline.
The numbers are the part that stopped me cold. Roughly 90% of people with this sleep disorder will go on to eventually develop Parkinson disease or dementia with Lewy bodies. Not “might.” Will, in the overwhelming majority of cases. What researchers still couldn’t predict, until recently, was which disease, and when. A 2025 study followed 250 patients with isolated RBD alongside 178 healthy controls for an average of six years, using an advanced MRI technique to track fluid clearance in the brain, and found that patients with a lower DTI-ALPS index in the left hemisphere of the brain were 2.4 times more likely to develop Parkinson disease in the years that followed.
Years, not months, ahead of the diagnosis
This is the detail that reframes the entire “wait and see” instinct families default to. RBD is generally considered to be associated with synucleinopathies, such as Parkinson’s disease, dementia with Lewy bodies, and multiple system atrophy, and usually precedes years before the first symptom of these diseases. Cognitive decline, when it eventually surfaces in people with RBD, doesn’t arrive as a sudden fog. Cognitive decline in iRBD begins up to 10 years prior to phenoconversion, according to a multicenter study published in Alzheimer’s & Dementia in 2023. Ten years. That’s the gap between “he acts out his dreams sometimes” and an actual diagnosis sitting on a chart.
And the first cognitive domains to slip aren’t the ones we associate with dementia at all. Forget forgotten names for a moment. Attention and executive dysfunction are the strongest predictors of dementia in iRBD. Researchers have also found that a comprehensive neurocognitive battery in iRBD patients often demonstrates reduced attention and executive functions, and later memory and visuospatial decline, starting even before the development of mild cognitive impairment. In practice, that can look like difficulty planning a route, hesitating over a familiar task, or losing the thread of a conversation, none of which scream “dementia” to a worried adult child scrolling symptom checklists at midnight.
There’s also a quieter cluster of clues that tend to show up around the same time as the sleep disturbance, easy to dismiss individually. Patients frequently present with hyposmia, altered taste perception, and color vision deficits, along with blood pressure drop when standing, constipation, urinary problems, and erectile dysfunction. None of these read as neurological on their own. Together, layered under a sleep disorder, they start forming a pattern a specialist recognizes instantly and a family, understandably, doesn’t.
Sleep architecture itself is the tell
RBD is the dramatic version of this story, the one with flailing limbs and shouted dream-dialogue. But the science behind sleep and cognitive decline runs deeper than one dramatic parasomnia. A 2024 study in the journal Sleep followed a cohort for a median of 12.8 years, and 75 patients developed a neurodegenerative disease which included Alzheimer’s disease, amnestic mild cognitive impairment, and vascular dementia, among others. The researchers found that reduced N3 and reduced REM sleep, as well as increased wake after sleep onset, each predicted increased risk for developing the broad grouping of neurodegenerative diseases, and this held true even after accounting for age, weight, and other sleep disorders. In plain terms: how a person sleeps, not just whether they sleep, carries information about brain health that shows up on a sleep study long before it shows up in a doctor’s office memory test.
Evening changes get dismissed constantly, and not without reason, since so-called “sundowning,” the late-afternoon confusion and agitation some dementia patients experience, is a well-documented but later-stage symptom. Restlessness, agitation, anxiety, tearfulness, and wandering especially in the late afternoon or evening are recognized signs, according to the National Institute on Aging, but they typically emerge once the disease has already progressed. RBD is different precisely because it shows up so early, often a full decade before anyone starts worrying about forgotten names, missed appointments, or repeated questions.
If there’s a nightly pattern in your own home that gets waved away with some version of “it’s just his age,” a sleep study is a low-stakes, high-information next step, one most neurologists can order without much friction. The father in my story eventually got his sleep study. The forgotten names came three years later, exactly as the specialist had quietly predicted the day she first asked how long the dream-shouting had been going on.
Sources : alz-journals.onlinelibrary.wiley.com | medlink.com