Hot off the press: Rest-activity rhythms predict cognitive function in Parkinson's disease

Last year I gave a talk to a peer support group of people with Parkinson’s disease (PD). I thought it would be a round table of maybe 10-12 people, but I ended up having to use a mic because I was talking to a ballroom of dozens more, including many family members of those with PD. It wasn’t because I’m famous or anything (hah!), but because the topic was important to them and rarely ever discussed by their neurologists: “Non-motor symptoms in Parkinson’s disease.” When I asked the room to shout out the PD symptom they hate the most, expecting to hear “tremor” or “problems with walking”…what I overwhelmingly heard instead was “sleep problems,” “thinking problems,” and “depression.”

These are some of the silent, non-motor symptoms of PD.

Photo by rawpixel.com from Pexels

Photo by rawpixel.com from Pexels

I’m taking a week off from regular tip-based programming to talk about this topic, which is the subject of my latest peer-reviewed scientific study, just accepted for publication last week at Movement Disorders Clinical Practice.

This study was part of my doctoral dissertation. The data come from a fabulous 2009-2011 study conducted by Dr. Karina Stavitsky Gilbert, who was a graduate student at Boston University in Dr. Alice Cronin-Golomb’s lab, which I was lucky enough to join years later. In this study, Dr. Stavitsky recruited 35 individuals diagnosed with PD and studied their cognitive function and sleep. Before I delve into the nitty-gritty of what I did to analyze her data from a fresh angle, here’s a PD 101:

Parkinson’s disease (PD) is a progressive neurodegenerative disorder, meaning it’s a disease that spreads through the brain and causes worse and worse symptoms over time. The most well-known (and easy to detect) symptoms of PD are motor symptoms—tremor in the limbs and head, rigidity of limbs, difficulty balancing and walking, and other problems of movement. This is why PD is usually thought of as a movement disorder.

However, the last couple of decades in PD research makes it clearer and clearer that there is a lot more going on in PD than problems with movement. For example, individuals with PD often lose their sense of smell years, sometimes decades, before the first tremors begin. Many experience depression, difficulty with thinking, or changes in their vision. Virtually all those with PD experience some prodromal symptoms—that is, symptoms that begin before being formally diagnosed with PD—without necessarily knowing that PD is the culprit.

Photo by rawpixel.com from Pexels

Photo by rawpixel.com from Pexels

One of the most common prodromal symptoms of PD is sleep disturbance.

(If you have sleep disturbance, this does not mean that you’re going to have PD later! I’m looking at you, insomniacs. The vast majority of causes for sleep problems are not brain disorders, so I wouldn’t add this to your list of sleep-related worries.)

Much research on sleep in PD has documented how sleep is related to other PD symptoms, particularly the non-motor symptoms. However, researchers have largely ignored a related, but uniquely important, process that is also impaired in PD—circadian function.

Circadian rhythms are the near-24-hour biological cycles that our brains and bodies go through to keep our machinery run smoothly. Virtually every system in your body runs on a circadian clock. So, if your circadian rhythms are thrown off, everything will be negatively affected.

Photo by Miguel Á. Padriñán from Pexels

Photo by Miguel Á. Padriñán from Pexels

And this is what I hypothesized to be happening in PD: disrupted circadian function was throwing a wrench in multiple biological systems, causing problems for mood, thinking, sleep, and other areas affected by PD.

To test this idea, I re-examined Dr. Stavitsky’s data from 2009-2011. She had estimated the PD participants’ sleep parameters using a device called an actigraph, which is like a scientifically fancier Fitbit. By analyzing the minute-to-minute movement data gathered by the actigraph in a fresh way (specifically: nonparametric circadian rhythm analysis), I was able to estimate some parameters of participants’ circadian function.

I’ll skip all the technical details of how I did this analysis—they’re not very fun.

What I found, with the help of my collaborators at Harvard Medical School and Boston University, was that PD participants’ interdaily stability—basically, a measure of how similarly their rest-activity patterns looked from day to day—was a significant predictor of their performance on attention, reaction speed, and spatial navigation. Importantly, I was able to confidently say that circadian function pulled its own weight in this analysis, and not just acting as a proxy for sleep. In fact, for the first time in a scientific study, I simultaneously looked at the contributions from both of these variables, and showed that circadian function actually did a better job of predicting cognitive function than sleep did.

This finding is important for a couple of reasons:

  1. For the first time, we can confidently say that circadian rhythms, separately from sleep, are associated with daytime symptoms in PD.

  2. Because of this, we now have a new intervention target. That is, now that we know circadian rest-activity instability is related to cognitive problems in PD, we have one more potential way to improve symptoms—by improving circadian function.

Photo by Pixabay from Pexels

Photo by Pixabay from Pexels

To sum up: This study found that circadian rhythm stability, as measured by the day-to-day patterns of rest and activity, predicts daytime cognitive functioning in those with Parkinson’s disease.

Last year, I also conducted a separate study that explored the day-to-day relationships between sleep and non-motor symptoms in PD. The most fun part: this was the first ever study to measure sleep and daily symptoms in Parkinson’s disease using smartphones. I’ll write a post on that study when it gets published. Meanwhile, sleep tight, everyone!