Activity trackers may not accurately track sleep

The increasingly popular wearable activity trackers may not be as accurate at monitoring sleep as compared to measuring steps and physical activity, a new study has found.
The study led by researchers from the University of North Carolina and RTI International, a US based non-profit organisation, found that the trackers are better at measuring some metrics than others.
Lead author Kelly Evenson 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 behaviours are growing significantly in popularity," said co-author Robert Furberg, senior clinical informaticist at RTI International.

"We conducted this review to understand how accurate these devices are," Furberg said.
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 over-estimate at slower speeds and under-estimate 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 under-estimated calories used, and over-estimated 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.
The study was published in the International Journal of Behavioural Nutrition and Physical Activity.