For one, there's no such thing as a typical resting heart rate. In fact, it can vary by as much as 70 beats per minute from one person to another, say researchers from the Scripps Research Translational Institute.
It doesn't end there. Our own heart rate can change with the seasons—it tends to be higher in January and lower in July—and even vary by 10 beats or more at any time.
But the biggest difference is between people, as the researchers discovered when they analysed data gleaned from sports watches that had been worn by more than 92,000 people for 320 days.
The resting rate varied by as much as 70 beats between the large group, and the researchers were at a loss to understand why. Factors of age, sex, body mass index (BMI) and sleep patterns together accounted for less than 10 percent of the variation—which means that nobody really knows why it differs by so much.