LodestarHeart rate variability, or HRV, is one of the most useful numbers your Fitbit records, and one of the easiest to misread. Here's what it is and how to make sense of it.
What HRV actually measures
Your heart doesn't beat like a metronome. Even at a steady 60 beats a minute, the gap between one beat and the next might be 0.95 seconds, then 1.05, then 0.98. HRV is a measure of that variation.
The variation comes mostly from your nervous system. When you're rested and relaxed, the "rest and digest" side is in charge and the gaps between beats vary more. When your body is under strain from hard training, poor sleep, alcohol, illness or stress, the "fight or flight" side takes over and the rhythm gets more even. So higher HRV, for you, usually means more recovered.
Fitbit reports HRV as RMSSD (the root mean square of successive differences between beats, in milliseconds), measured while you sleep. Measuring at night matters: it's the one time your body is still and conditions are similar from one day to the next.
What's a normal HRV?
This is where most people go wrong. Healthy adults can have nightly RMSSD anywhere from the teens to over 100 milliseconds. It drops with age, and it varies a lot between people for reasons that have nothing to do with fitness. Your number compared with someone else's tells you very little.
What does tell you something is your number compared with your own normal. Sports-science research on HRV-guided training uses exactly this approach: a rolling average of your recent nights, compared with your longer-term baseline, with small day-to-day wobbles ignored (Plews and colleagues, Sports Medicine, 2013).
Why your HRV goes up and down
- Alcohol is one of the most reliable ways to lower HRV that night, even in moderate amounts.
- Hard training often dips HRV for a day or two, then it rebounds as you adapt.
- Feeling run down or unwell can come with lower HRV and a higher resting heart rate, though many other things cause the same change.
- Short or broken sleep, late meals, heat and stress all tend to pull it down.
- Single nights are noisy. One low reading on its own means little. Two or three in a row, especially with a higher resting heart rate, is worth paying attention to.
How to read it day to day
Look at the trend, not the number. A few practical rules:
- Compare last night with your average over the past week or so, not with yesterday.
- Treat changes within your usual range as noise.
- Pay attention when HRV and resting heart rate move in opposite directions at the same time. Lower HRV plus higher resting heart rate is a stronger signal than either one alone.
How Lodestar uses it
Lodestar is an iPhone app that reads your Fitbit data through Google Health and does this comparison for you. Your Recovery score weighs last night's HRV and resting heart rate against your own rolling baseline, plus how you slept. Tap the score and it shows exactly how many points each one added or took away. Everything is worked out on your phone.