I found the seasonal variation really interesting, to summarise on average people's resting heart rate is ~2bpm higher at the start of the year than it is in Ju

I found the seasonal variation really interesting, to summarise on average people's resting heart rate is ~2bpm higher at the start of the year than it is in July. The authors don't hypothesise why this is, my guess would be general unhealthy behaviour like excess eating, sedentary behaviour and alcohol consumption around the Christmas period (population was from the US), but it could also be colder ambient temperatures. It'd be interesting to try and control for this e.g. see if this phenomenon is also present in non-Christian majority countries, or Christian countries within the tropics where the annual temperature variation is smaller, but on average higher than most of the US.