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I've been using a Fitbit to track my resting heartrate (among other things). The value has consistently been in the 55-60 range for several weeks (most typical value over the past week or two is 57). Over the past few days, however, it jumped to around 62; a 10% increase.

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The bulk of the increase occurred 2 days ago, and the only thing I can think of that happened within that timeframe is that I got a mild sunburn (from kiteboarding, so primarily on my face/forehead from looking up at the sky all afternoon). But correlation doesn't necessarily mean causation. So my question is, can a sunburn lead to an increase in the body's resting heartrate? And if so, why?

Misc Notes

I know a Fitbit isn't a medically accurate device, and that there's pending litigation over whether they're accurate at all. However my anecdotal experience with my particular device is that it's quite accurate when at rest (during strenuous exercise the instantaneous readings can indeed be a bit funky and it can temporarily lose count altogether; although the average values that get logged to the dashboard seem reasonable despite the transient outliers/glitches). So I don't think device inaccuracy/margin of error is the explanation.

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  • Where you also outside and possibly more active than usual when you were getting the sunburn? How about warmer? Both of these easily raise your heart rate. Would you mind editing out " or other minor injury or illness" as that makes this overly broad.
    – Atl LED
    Jan 26, 2016 at 2:35
  • @AtlLED - Maybe slightly more active than usual, but only for that one day. Subsequent days were 'normal', and didn't cause the rate to drop back down. And warmer, yes, it has been a fair bit warmer over the past week, particularly overnight and in the mornings. Which is probably when I have the bulk of my 'at rest' time.
    – aroth
    Jan 26, 2016 at 4:13
  • After a bad burn (pale skin plus Australian summer) my resting hearting on a Fitbit went from 49 up to 53 and down again over the course of the week while the burn subsided. (Daily exercise levels remained consistent, you don’t get to 49 without it being so, hydration was good, sleep temps stable due to Aircon etc). Def seems to be that inflammation due to the burn seems to raise it a few points.
    – Tom
    Jan 28, 2018 at 22:23

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One word: dehydration.

You spent a day out in the sun, exerting yourself, and probably didn't maintain your normal level of hydration. Consequently, you're mildly dehydrated (or, more properly, hypohydrated).

Dehydration means a lower blood volume, which in turn means your heart has to beat faster to move the same volume of blood. Consequently, your heart rate is mildly elevated until you restore your normal hydration levels.

Summary of Findings 1

  • Scientific Definitions: Hypohydration is the steady state condition of decreased total body water. Dehydration is the process of losing body water (eg, during exercise).
  • Body mass losses as small as 2% have been shown to result in an increase in cardiovascular strain and subsequently decrease performance during exercise. Exercise in the heat further exacerbates cardiovascular strain, thus causing further decrements in performance.
  • Results have shown that for every 1% decrease in body mass during exercise in the heat there is an increase in heart rate of 3.29 beats/min. This equates to an increase in heart rate of 10 beats/min if an athlete is 3% dehydrated.
  • Exercise at a fixed and variable intensity has shown an increase in heart rate of 3.55 and 1.39 beats/min respectively during exercise in the heat.
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