Your wearable shows an HRV of 42 ms this morning. Yesterday it was 58. Start with context, not a verdict: compare your usual pattern, sleep and how you feel before deciding what to do. 3

What HRV actually measures

Heart rate variability is variation in the intervals between consecutive heartbeats. Common measures such as RMSSD summarise differences between neighbouring intervals in milliseconds. 1

Heart rate usually speeds up during inhalation and slows during exhalation. Breathing is one influence on HRV, not its only cause. 1

RMSSD reflects mainly parasympathetic influences under typical resting conditions. It does not directly measure the balance of your whole nervous system. Low HRV does not prove you are stuck in fight-or-flight mode. 1

There is no universal good number. Age, health and fitness affect HRV. For everyday tracking, your usual range is more useful than comparing 35 ms in one person with 90 ms in another. 3

Why wearable numbers differ

WHOOP, Oura and Garmin do not summarise the night identically:

  • WHOOP: its Recovery documentation describes a weighted overnight average, emphasising slow-wave sleep and later parts of the night. 2
  • Oura: the displayed overnight average is the mean of five-minute samples collected during sleep. 3
  • Garmin: compatible watches record overnight HRV and compare a seven-day average with an individual baseline, initially established over about three weeks. 4

These are manufacturers' descriptions, not proof that one method is best. Keep comparisons within one device and measurement routine rather than treating the brands' readings as interchangeable. 2 3 4

An unusually high reading is not automatically better. Garmin can flag averages above, as well as below, your baseline as unbalanced. 4

Movement, signal errors and irregular beats can distort HRV. Optical wearables estimate beat timing from pulse signals rather than recording the heart's electrical activity as an ECG does. 1

What moves HRV up and down

Treat sleep, training and stress as context to record, not explanations the app has proved. Oura links lower readings with stress and poorer recovery, but a low number cannot identify the cause. 3

Alcohol has clearer supporting evidence. In an observational study of 4,098 adults, drinking was associated with lower HRV during the first hours of sleep, with larger changes at higher intakes. 5

Exact drinking times were unknown. The study cannot establish that one or two NZ standard drinks within four hours of bed always suppress HRV, or predict when your reading will recover. 5

If you already drink, compare alcohol-free evenings with your existing pattern. Do not start drinking to test the device, or interpret an unchanged score as proof that alcohol had no effect.

Exercise can improve HRV over time. A small trial in 66 sedentary middle-aged adults found improvements after several different 12-week exercise programmes. 6

That does not establish Zone 2 as uniquely effective, or promise a higher score after adding one session. Choose activity you can repeat and recover from. 6

Breathing can change the measurement itself. A rise during slow breathing does not establish better overnight recovery or long-term health. Keep deliberate breathing exercises separate from baseline readings. 1

For your notes, include late meals, missed meals, fluid intake, deadlines and disrupted sleep. If a large evening meal feels uncomfortable, try an earlier meal rather than assuming protein or fat is the problem.

Keep adequate fuelling, regular sleep, daylight and time outdoors in your routine. Do not make their value depend on tomorrow's HRV, or treat extra protein, nasal breathing or morning sunlight as guaranteed score-raising tools.

How to read your trend

Start with your device's baseline process. A 30-day view is a practical review window, not a medical requirement. Garmin, for example, uses about three weeks to establish its initial range. 4

First, establish your usual pattern. Wear the device consistently. Note both the average and how much readings vary during ordinary weeks, rather than selecting only your best nights.

For waking spot checks, use the same time, posture and recording length after settling quietly. These conditions affect comparability; sleep is not the only valid measurement setting. 1

Second, inspect deviations. Do not apply a universal percentage cutoff. Neither a single low day nor a fixed number of low days identifies illness or overtraining. Use the device's range as context, not a diagnosis. 2 4

Third, look at the response. After a hard session or late night, record what happens alongside tiredness and performance. Ask whether repeated patterns suggest a useful change, rather than grading your resilience by recovery speed.

For example, after several late work nights, review your sleep opportunity and workload first. Avoid changing food, exercise and bedtime simultaneously if you want to learn which adjustment helps.

Where HRV is useful, and where it is not

Use HRV to prompt questions: did this training week feel harder, did alcohol-free evenings differ, or did a steadier routine coincide with feeling better? Treat these observations as clues, not controlled experiments.

Illness research is promising but limited. One Apple Watch study found changes in daily HRV patterns before COVID-19 diagnosis. Only 13 participants received a new diagnosis during follow-up. 7

It analysed daily patterns, not simply a low overnight number. It does not validate a reliable 24-to-48-hour warning for a cold, or let a normal score rule out infection. 7

For training, do not cancel automatically after one low reading. Review sleep, symptoms and recent workload. Consider an easier session when you also feel unusually tired or unwell; do not let a reassuring score override symptoms.

Keep HRV as an optional input, not a reason to order a package of blood tests or scans. For a broader tracking framework, see building your personal health dashboard.

The honest limitation

If checking HRV makes you worry or changes how you judge an otherwise good day, try reviewing it weekly instead. Hide readiness notifications or take a break. Tracking should serve a purpose you find useful.

Discuss persistent unexplained changes with your GP if you are concerned, particularly alongside new symptoms. Bring the trend and symptom notes rather than requesting thyroid or iron tests solely because HRV fell.

Do not wait for a trend if you have urgent symptoms. In NZ, call 111 for chest pain, difficulty breathing or fainting, regardless of the wearable's score. 8

If you are unsure how urgently to seek help, call Healthline on 0800 611 116. 8

What to do this week

  • Find your device's usual range and recent trend. If you are new, prioritise consistent collection rather than interpretation.
  • Record one practical change for seven days, such as alcohol-free evenings or steadier bedtime. Include how you feel; repeat before drawing conclusions.
  • Choose manageable activity, such as a walk or easy cycle. Build gradually rather than adding exercise to chase HRV. See Zone 2 training explained.
  • Decide when you will review the data. Weekly may be enough. Make the next decision from the whole picture, not the colour of a recovery score.