Q&A: Why does my resting heart rate fluctuate so much? Am I really losing/gaining cardiovascular fitness?

Why does my resting heart rate fluctuate so much? Am I really losing/gaining cardiovascular fitness in a matter of days or weeks, like my fitness tracker suggests?

Resting heart rate can be affected by many factors, so fluctuations are not unusual. If you notice that your resting heart rate is higher than usual, consider if any of the following conditions might account for this change.

1. Temperature:
Summer is coming, and if the environmental conditions are hot and humid, your body will need to dissipate heat. The blood vessels to your skin will dilate and your heart will have to pump a little faster to circulate blood to the surface of your body.

2. Dehydration:
If you’re prepping for a fall marathon, you may be ramping up your mileage in the heat of the summer. If you’re not keeping up with your fluid losses, you may end up a little dehydrated, which means that your blood plasma volume will be lower than ideal. This drop in blood volume will cause your heart to beat a little faster.

3. Exercise Intensity:
If you’ve been training hard—either on the roads or in the weight room—your body’s recovery processes will be working to repair muscle damage, build new muscle, and replenish glycogen stores. All of this metabolic work results in an increase in heart rate.

4. Mental or physical stress:
Work demands, family commitments, difficult relationships, inadequate sleep, marathon training, or the onset of a cold or flu can all lead to an increase in levels of stress hormones and a higher-than-usual resting heart rate.

5. Diet or Medication:
Caffeine, alcohol, certain herbal supplements, and some medications can all affect heart rate.

Check your resting heart rate under consistent conditions—aim for first thing in the morning while you’re sitting or lying quietly. if you notice a higher than usual number, consider what your body may be trying to tell you.

Do you need to drink more water, incorporate more recovery into your training plan, get more sleep, cut down on caffeine, or just turn on the AC?  Your heart rate is sensitive to both your external and internal environment, and can be your first clue that something is stressing your system—a gentle reminder to take better care of yourself.

Your resting heart rate should only vary by 5 to 10 beats, so if it is unusually slow or very rapid, if it’s irregular, or if you feel weak, dizzy, or faint in conjunction with the change in heart rate, you should seek medical attention.


Polly de Mille

Polly de Mille, RN, MA, RCEP, CSCS, USAT 1, is the coordinator of performance services at the Tisch Sports Performance Center at Hospital for Special Surgery. In addition to being a registered nurse, she holds a master’s degree in exercise physiology and is certified by the American College of Sports Medicine as a registered clinical exercise physiologist, exercise specialist, and exercise test technologist. She is also a certified USAT Level 1 triathlon coach.

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