Every person has an inner clock, called a circadian rhythm, that ensures each physiological process is optimized depending on the time of day.

This rhythm dictates our sleep, hormones, eating and digestion habits, and body temperature. It can play an important role in overall health, according to the National Institute of General Medical Sciences.

Scientists are interested in learning more about circadian rhythms in order to help people know the best times of day to sleep, eat, exercise or take medications. But traditionally these studies involve participants living in a time-isolation clinic for several days, where researchers control light and dark periods.

But now researchers from the University of Michigan propose a new method to understand people’s circadian rhythms using wearable smartwatches.

"I think a big question has been, 'Can we measure circadian rhythms with wearables, and how can we do that?'" Daniel Forger, a research professor of computational medicine and bioinformatics at Michigan Medicine, said in a statement.

To do so, Forger and his colleagues created a statistical method that extracts a person's circadian rhythm based on heart rate data provided by their smartwatch.

TOP-LINE DATA

Using more than 130,000 days of real-world data collected from the smartwatches of 900 medical interns on rotating shift schedules, the researchers found distinct and individualized circadian rhythms for the participants that were consistent with clinical data.

The researchers’ algorithm uses smartwatch data such as heart rate, activity, meals, posture and stress to estimate circadian rhythms. They say the algorithm gives a better look into real-world stimuli that could affect the rhythms.

"We've shown that you can take a wearable signal and directly measure circadian rhythms in the real world, and the real world has so many things that affect circadian rhythms that you aren't going to measure in the lab," Forger said. "There are some scenarios that you have to always be a little more careful with, using real-world data. But again we're going to pick up other things you may not experience in a lab."

An example from the study showed one participant’s heart rate quickly adjust to their changing work schedule, which demonstrated that their circadian rhythm was able to adapt to a bedtime and waking time that were almost the opposite of what they had previously been experiencing, according to the researchers.

The researchers say the ability to use wearable data to understand circadian rhythms can give both scientists and users insights into how to use that information to improve health.

"Measuring that signal not only provides information about the body's circadian timing, but also characterizes how each individual's heart rate behaves," said Clark Bowman, a professor of mathematics and statistics at Hamilton College and a researcher for the study.

"We can use this information to track how the body adjusts to new schedules, study how physical activity affects each individual's heart rate slightly differently, and even quantify the effect of being active at different times of day on the body's internal clock. Smart watch users could have real-time information on their circadian clock to help adjust to jet lag or shift work, manage circadian disorders, or identify abnormalities in heart rate, which might present health risks."  

Understanding the potential benefits this capability could have for consumers, the team created a smartphone app where users upload their wearable data to get reports on their circadian rhythms. Called Social Rhythms, the app is available for download on iPhone and Android devices.

METHODS

Participants in this study are also a part of an ongoing research initiative called the Intern Health Study. They worked both day and night shifts, making them ideal candidates to study circadian rhythms, and they continuously wore smartwatches from Fitbit, Apple or ​​Xiaomi.

The majority of participants were female, and most were between 25 and 30 years old.

To estimate circadian rhythms, the team’s algorithm discards sleep data and focuses on metrics collected during waking hours. From there, it takes into account whether a person’s heart rate is affected by activity, posture or meals to extract the underlying circadian rhythm.

THE LARGER TREND

Understanding people’s sleep and wake cycles is an important insight into overall health that many others are trying to decipher. While this study used a person’s waking hours to draw conclusions about circadian rhythms, others track people’s sleeping habits.

For example, Sleep Number has a smart bed and a connected app that together give users monthly wellness reports, circadian sleep analytics and a heart-rate-variability measurement.

Garmin works with Sleeprate to give users insights into how workouts and nightly sleep affect one another. Fitbit and Apple smartwatches both independently track and give sleep reports to their users.