The short answer
HRV is a clue about the regulation of your heartbeat, not your condition itself. Rather than reading "low = tired" and "high = recovered", treat a move from your own usual range as a hypothesis, and read it together with sleep, resting heart rate, exercise, circumstances and your own sense of things.
Key points
- HRV and fatigue or stress can be related, but not one to one
- The value moves with breathing, posture, time of day, exercise, sleep and alcohol
- Your own long-run trend, gathered under similar conditions, is the reference point
- On days when the number and how you feel disagree, do not dismiss how you feel
HRV reflects one part of the regulation
The interval between beats is influenced by the autonomic nervous system, breathing, blood-pressure regulation, hormones, temperature and activity. HRV puts a number on the fluctuation that results — it does not directly measure mood, stress, illness or recovery.
Research reports associations between HRV and many aspects of health and daily life. Identifying a cause from one person's single day is a different matter entirely.
Why compare with your own past
HRV varies widely between individuals and changes with age and with measurement method. It also moves within one person when the time, the posture, the depth of breathing or the activity beforehand changes.
So the reference is your own record, gathered on the same device at similar times in similar states. Watching a moving average or a usual range, rather than single values, keeps you from over-reacting to a passing wobble.
Four ways a morning can read
HRV low, and you feel tired
Look back at sleep, training load, alcohol and how you have been. It is useful for deciding to keep something in reserve today, but it does not establish a cause.
HRV low, but you feel fine
It may be measurement conditions or a single wobble. Do not override how you feel; watch what follows.
HRV normal, but you feel bad
Do not push through because of a number. Put symptoms and rest first, and see a clinician if you need to.
HRV high, and things feel good
This is not a guarantee against illness or of results. Use it to record what the conditions of a good day looked like.
Worth reading alongside
- Sleep duration, interruptions, how rested you feel on waking
- Resting heart rate and the trend in overnight heart rate
- Yesterday's activity and how recovered your muscles feel
- Alcohol, meal timing, caffeine, travel and night shifts
- Mood, pain, fever, medication changes — the things only you know
A correlation is not "it worked"
Even with a record showing "HRV was higher the day after I exercised", sleep, a day off and how much you drank may all have coincided. What a record shows first is association. It cannot establish that exercise raised your HRV.
HRV is easier to live with when you treat it not as a device that produces answers, but as one that produces better questions about your own days.
References
- HealthKit heart rate variability (SDNN) specification — Apple Developer
- Consumer Wearable Health and Fitness Technology in Cardiovascular Medicine — Journal of the American College of Cardiology
- Heart rate variability measurement and influencing factors — European Heart Journal — Digital Health
- Guidelines for rigor and reproducibility of HRV in cardiovascular research — American Journal of Physiology