Reading Heart Rate Variability Properly
HRV is the variation in time between consecutive heartbeats, and it tracks how much your parasympathetic nervous system, the recovery side, is currently in charge. It's genuinely informative, and also the most commonly misread number in consumer health. Three things matter. First, it's meaningless as an absolute: healthy adults span a range wide enough that your neighbour's normal could be your emergency, so it only means anything against your own baseline. Second, it moves enormously across a single day, which is why research protocols and serious tools use overnight or waking readings rather than whatever the watch caught in the middle of the afternoon.
Third, one night tells you very little. Plews and colleagues showed a rolling weekly average tracks genuine training adaptation where single readings mostly track noise. The useful question isn't "is my HRV good" but "has my weekly average moved meaningfully against my normal range".
HRV feeds Kriterion's daily readiness score against your own baseline, never against somebody else's, and it stays quiet until it has enough nights to have a baseline at all.
References
- Plews DJ, Laursen PB, Stanley J, Kilding AE, Buchheit M. Training adaptation and heart rate variability in elite endurance athletes: opening the door to effective monitoring. Sports Medicine, 2013.
- Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. Heart rate variability: standards of measurement, physiological interpretation, and clinical use. Circulation, 1996.
- Buchheit M. Monitoring training status with HR measures: do all roads lead to Rome? Frontiers in Physiology, 2014.
- Al Haddad H, Laursen PB, Chollet D, Ahmaidi S, Buchheit M. Reliability of resting and postexercise heart rate measures. International Journal of Sports Medicine, 2011.
Kriterion is the app this reasoning is built into
It measures your maintenance calories from what you actually ate against what the scale actually did, tells you how firmly the number is pinned down, and checks its own past predictions against your results. Free logging, UK food data, and no ads.
Join the list and you get the founding offer — the launch price, held for as long as you stay subscribed. It is capped at the first 1,000 subscribers or 90 days from launch, whichever comes first, and it will not be extended. One confirmation now, one email on launch day. Not a newsletter.