It’s fair to say that mathematicians rarely get to work with bears. “Wow, that’s kind of cool and different,” said Associate Professor of Mathematics Tanya Leise on being asked to co-author a study on whether their circadian clocks “keep ticking” during their winter dormancy.
The 2016 study, published in Frontiers in Zoology, took place at the Washington State University Bear Center, where grizzly bears (orphaned and unable to live in the wild) can be analyzed in captivity. Researchers also culled data from wild bears denning nearby.
The takeaway is in the study’s title: “The Bear Circadian Clock Doesn’t ‘Sleep’ During Winter Dormancy.” In other words, Leise and her fellow researchers found that daily biological processes do not grind to a halt, as was once
assumed.
They determined this by equipping the captive bears with monitors (think: big Fitbits) that measured their movement, and by logging the bears’ body temperatures and responses to light. Plus, they examined cell cultures taken from the captive bears.
This data gathered in captivity is, for humans, captivating in its implications: For the past hundred years, we have lived in an unprecedented era of light disruption of our own circadian clocks, which are tied to the light/dark cycle. The 20th century brought us the electric grid. In the 21st, there’s also the ever-present light of computer and smartphone screens.
Scientists speculate that this light disruption—plus circadian-altering phenomena such as jet lag and shift work—may be tied to rising rates of obesity, diabetes, depression and bipolar disorder.
Indeed, circadian rhythms help regulate patterns of hormone production, cell regeneration and other daily biological processes. Millions of little circadian clocks, on the molecular level, take their cue from a master clock in the brain—the suprachiasmatic nucleus (SCN)—which tries to pinpoint when dawn and dusk are happening, and sends that information to the rest of the body.
Why did the WSU bear project, a biological study, require a mathematician? “The data is getting complex enough,” Leise says, “that it’s hard to interpret at face value.”
She crunched the numbers using “robust wavelet transforms,” a mathematical technique that detects frequencies in time series. In a musical recording, for example, wavelet transforms would pinpoint the sequence of notes being played. With the bears, it was all about gauging frequency of movement over the course of a day, and how this related to light exposure. Leise also created and analyzed models that use probability distributions and statistical inferences across a spectrum of data.
Leise is firmly in the inner circle of circadian research, with 21 publications on the subject. Currently, she’s most excited by one hypothesis, backed by her data, about how the rhythms get synchronized in the body. The process seems to be much more complex than the SCN directing from the top down.
To explain the hypothesis, Leise says it helps to picture cells as pendulum clocks in a room: “Initially, they’re all swinging together, but over time, some stop swinging wide, others go still, some swing out of alignment. Normally, you’d connect all the clocks to a rope so they’d swing in sync (like the SCN giving out direct orders). But my analysis is that some cells also ‘kick’ other cells so they’re not so tight together. To reset the clocks, you need a little messiness.”
Grasping how these clocks work in other animals is a big step toward developing therapies to fix them in humans. Which is why, when it comes to circadian patterns, Leise keeps bearing down.