Heart Rate And The Time Of Day

So my current plan is base training- a period of slow easy running that leaves me fairly fresh at the end of each session. For this I use my HRM as a guide to exercise intensity and to tell me when me to slow down. So I have been paying close attention to my heart rate than normal and to my surprise have found quite a variation between morning and afternoon.

The past four runs have been at roughly the same pace but the average heart rates have been: 146, 156, 150, and 145 bpm. The middle two runs were in the afternoon, the other two in the morning. The morning runs were at a lower rate yet the perceived exercise level was the same. There has to be a slight caveat as they were four different routes (and the 156 route was probably a bit more hilly than the others) but they were similar enough to make comparisons . I can think of no other confounding factors as the time between eating and running was about the same in all cases and certainly temperature was not an issue (as it might be in the summer where afternoon heat can raise the heart rate).

Now four runs are not enough to draw any conclusions but they do indicate that there might be something going on. It is probably not important, just a part of normal diurnal rhythms, but it is of passing interest - not because it makes any difference to training but because it is slightly counter intuitive. The evidence point to the afternoon as the time of peak athletic performance (just look at the time of day records are set) and one would therefore assume that when the body is working at maximum efficiency, for any given speed would require proportionately less effort and hence need a lower heart rate. Instead it looks as if everything revs up a bit during the course of the day, including the heart for the same level of output.

Rootling around on Google I found a newspaper article by someone who had observed exactly the same pattern (the accompanying photos raised the additional question of why her dress sense was also worse in the morning). But there are also straightforward articles that describe the factors that can affect heart rate and include the time of day. From the world of running there is this from Peter Pfitzinger, and from world of professional football referees (not a phrase I thought I would ever use in this blog) this.

But the body is never simple and this very small scale study on cyclists found that although the time of day affected performance it did not affect heart rate. Now a sample size of 8 is not much larger than my single anecdote, nevertheless it does indicate that the diurnal effect might not be quite so straightforward.

The key point though is does it matter?

In practical terms No - the heart rate is a minor detail in the circadian pattern that makes no difference to when or how you should train. Even the larger circadian changes should not effect the time you like to train (as was concluded in  this paper). 

However it does show that using the heart rate as a proxy for the level of effort should be treated with caution. As a rough indicator, where a few beat either way are neither here nor there, it is fine but if followed too rigidly it could be misleading and give a false sense of certainty about what the body is doing. Almost all the articles I have read about heart rate training have treated the numbers as stable and reliable (eg you have a definite maximum heart rate for each type of exercise) whereas they not only vary at different times they fluctuate daily.

This page from the 'Physiology of sport and exercise' has an interesting table of the ways heart rates can vary. Never mind the time of day the most startling factor is sleep. The difference between 8 hours and six hours could be 10 beats - now that might well be something I should pay attention to.