Resting Heart Rate – Settling Down

I finally learned how to do error bars in Microsoft Excel! In honor of that lesson, I wanted to share a graph that I find very interesting.

I believe I have written before about not loving how FitBit determines resting heart rate, but I never really understood what was happening until recently. I started obtaining my own daily resting heart rate value from my FitBit data, looking at my heart rate minimum right before waking up. I have been tracking both my approximation of my resting heart rate and the FitBit designated resting heart rate.

In doing this, I believe that FitBit uses a smoothing function as part of their algorithm. My Fitbit resting heart rate data never seems to be independently based upon the current day, but rather related more to my recent history. I have noticed that it does go up and down over time, but it’s not always perfectly correlated to the data. This helps me see why some days, my heart rate was relatively low before waking up, but my FitBit resting heart rate did not capture this.

My FitBit determined resting heart rate has been decreasing over time, but, I feel my personally obtained resting heart rate value shows the trend better, so that is the data that I will be sharing. I have binned the data into weeks. Week 0 is before starting my fasting experiment in January – that bin comprises 11 days (days -10 through and including day 0). Weeks 1-9 are data bins each consisting of 7 data points for the week. Binning the data into weeks allows me to calculate the standard deviation of the measurements within each bin, which I can use for my error bars. The error bars are +1 standard deviation and -1 standard deviation of each week’s data, so approximately a 95% coverage probability.

The binned data is below, as well as the graph:


Average Resting Heart Rate Standard Deviation


77.3 3.3


80.7 5.1


80.1 2.2


78.0 2.6




5 77.3


6 70.7


7 70.1


8 68.1


9 71.1



I found the data interesting, although not everything I am going to share as interesting is statistically significant, but my main point is. I first noticed that my resting heart rate increased when I started the fast. In fact, I saw that my overall heart rate increased slightly during the fast, and even increases after approximately 24-36 hours of fasting for shorter fasts I have done intermittently. I guess that’s evidence to show that my body is revving up and working harder while I fast.

Ultimately, I want my body to be efficient and I want to be more “fit” from a cardiovascular level as time goes on. That means I was hoping to see a decrease in my resting heart rate. After my initial fast, my resting heart rate slowly started to come down, but it was a gradual process. However, between week 5 and week 6, there was a significant drop and it’s been stable ever since.

I honestly don’t know what caused the sudden drop in resting heart rate, but I am excited to see it. I have always had a higher resting heart rate than many people I know and I never understood why. Even when I was super active with physical fitness, my FitBit resting heart rate was higher than I felt it should be. But, now, it’s hanging out in a much nicer place – and really the only exercise I am doing is yoga and Body Flow. I’m trying to stick to body weight based strength, flexibility, and relaxation.

I’ll keep measuring, but I like the big jump, and it goes to show that sometimes results just take a little time to catch up with healthy habits.

One thought on “Resting Heart Rate – Settling Down

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