You probably know someone using a fitness tracker. Recent models track daily heart rate and sleep. With just a few years of such models in the market, the companies collecting these data anonymously can provide insights that we simply did not have the data for just 5 years ago.

Here is such a summary from fitbit data.

Among the highlights are that 7 and a bit hours of sleep seem optimal for having a low standing heart rate, that Italians have a very low resting heart rate after controlling for daily activity while Indians have a very high, and life events (like surgeries or family gatherings) can be identified by looking at resting heart rate variation throughout the year.