Out-of-whack body clock causes more than sleepiness

When the body’s “clock” doesn’t match the cues its getting from outside, people can feel bad. Researchers are using math to explain this “circadian-time sickness.”

If you’ve ever flown across an ocean or continent, you’ve likely felt jetlag. It may occur when your brain thinks it’s night but you’re staring at a clear bright sky. Such a mismatch can be painful. It can cause sleepiness. It even can trigger nausea and more. Now, scientists are using math to understand why.  

People who travel between time zones or work night shifts often feel this type of time confusion. And many studies have shown that a screwed up circadian clock can cause health problems. In addition to sleep issues, researchers have found depression, metabolic syndrome and memory troubles.

But despite these symptoms, scientists don’t have a good understanding of how or why they develop, says Raymond van Ee. He’s a neuroscientist at Radboud University in the Netherlands.

He and his colleagues suggest that all of these different symptoms can be part of the same ailment.

These symptoms, they note, have much in common with motion sickness. (That’s when you become queasy if movements you feel don’t match what your eyes see.) So the researchers suggest naming the new ailment “circadian-time sickness.” And math may explain this condition, they argue.

Van Ee and his team used something called Bayesian inference to describe what happens when the body and brain get out of sync. In statistics, inference looks at how likely it is that two different things are connected. It uses math to interpret how believable those links are. Bayesian inference, especially, does this by combining prior knowledge with new information. Our knowledge is always being updated to fit in as new findings and developments occur. That way the brain can be constantly updated to make conclusions based on these new data.

The same math may explain how the brain makes and refines predictions about the world, van Ee says. The brain takes what it already knows and adds new information from the eyes, ears and rest of the body. Then it guesses where it is in space and time. The researchers believe that people get time sick when these cues don’t match up.

Their findings will appear soon in Trends in Neurosciences.

The idea is “intriguing and thought-provoking,” says Samer Hattar. He’s a neuroscientist at Johns Hopkins University, in Baltimore, Md. “They really came up with an interesting idea of how to explain the mismatch,” he says.

Nerve cells called pacemakers help the body keep track of time. Some pacemaking cells respond directly to light. That allows them to keep tabs on the outside environment. Other pacemaker cells rely on internal signals. These two groups of nerve cells can work together to set the body’s cycles without the need for some master biological clock. But it can cause a problem when the two timekeepers disagree. Their conflict confuses the body. And that can trigger health troubles, van Ee and his team now conclude.

Screwy sleep cycles may be the most common symptom of a mixed up internal clock. Yet one thing that makes the new description of time sickness so interesting is that it leaves out sleep. It’s true that shifted sleep cycles can cause problems. But the researchers believe that if these two sets of timekeepers disagree, someone might sleep just fine yet still feel sick.

This goes against the idea that out-of-sync rhythms cause poor sleep, which in turn affects both body and brain. That idea “was totally linear and beautiful,” Hattar says. “But once you start looking very carefully at the data,” he says, “you find inconsistencies that people ignored.”

It’s difficult to tease out the effects of sleep from other problems caused by out-of-sync rhythms, says neuroscientist Ilia Karatsoreos. He’s at Washington State University in Pullman. His research and that by others have shown that even when sleep is normal, mismatched cycles can still affect someone’s health. This new paper shows why it’s important to study and understand how all of the pieces fit together, he says.

The idea of time sickness still needs to be tested, Karatsoreos says. Yet it’s a “useful way for us to talk about this general problem, if only for the fact that it’s a way of thinking that I’ve really never seen before.”