Activity trackers are better at counting steps than measuring sleep, study finds

December 18, 2015

Wearable activity trackers that promise to monitor physical activity, sleep and more are becoming increasingly popular with health-conscious consumers. A recent study led by researchers from the UNC Gillings School of Global Public Health and RTI International found that the trackers are better at measuring some metrics than others.

Kelly Evenson

Dr. Kelly Evenson

Kelly Evenson, PhD, is a research professor of epidemiology at the Gillings School and an RTI University Scholar. She is the lead author of a new study titled, “Systematic review of the validity and reliability of consumer-wearable activity trackers,” published online Dec. 18 by the International Journal of Behavioral Nutrition and Physical Activity.

Evenson and co-authors conducted a systematic review of 22 published articles researching the ability of Fitbit and Jawbone – two popular activity trackers – to measure steps, distance, physical activity, calories and sleep.

“Wearable devices that track physical activity, sleep and other behaviors are growing significantly in popularity,” said Robert Furberg, PhD, senior clinical informaticist at RTI International and co-author of the study. “We conducted this review to understand how accurate these devices are.”

Several studies indicated that the step counting feature was accurate both in the lab and in the field. Only one study assessed distance tracking for the Fitbit, finding that the device tends to overestimate at slower speeds and underestimate at faster speeds.

Two field-based studies compared accelerometry-assessed physical activity to results from the trackers, with one study finding high correlation (in Fitbits) while another study noted a wide range in correlation (in both Fitbit and Jawbone brands).

Using several different comparison measures, other researchers found that both tracker brands over- and underestimated calories used, and overestimated total sleep time.

Overall, the systematic review indicated higher validity of step counting, inconclusive findings (based on few studies) for distance and physical activity, and lower validity for calories (energy expenditure) and sleep.

To make trackers as accurate as possible, the authors suggest some strategies for device wearers.

“When researching information on the trackers, we learned several tips users may be able to implement to make their tracker more accurate,” Evenson said. Helpful recommendations include:

  • Wear the tracker in the same position each day.
  • Enter personal details like height and weight correctly at initial set-up, and update if there is significant change in weight.
  • If the tracker provides these options, then:
    • Correctly calibrate the length of a walking stride.
    • Add more information via the device’s journal function.
    • Interact with the sleep mode settings.

Evenson conducted this systematic review as an RTI University Scholar. The RTI University Scholars Program provides support for talented academic researchers to spend one year of scholarly leave working in partnership with RTI researchers.

Gillings School of Global Public Health contact: David Pesci, director of communications, (919) 962-2600 or

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