Should I run today? Read the night, then choose the gear

What does a readiness score actually measure?

A readiness score compares overnight and morning signals with your own recent normal: usually heart rate variability (HRV, the beat-to-beat variation in your heart rhythm), resting heart rate and sleep, sometimes breathing rate, wrist temperature and recent training. None measures fitness or fatigue directly.

HRV moves with training, but also with alcohol, a late meal, a hot bedroom, work stress, an oncoming illness and the moment of the sample; resting heart rate drifts up with many of the same. A blended score is steadier, but still an estimate from the outside.

Apple's HealthKit documentation states that Health stores HRV as SDNN, the standard deviation of the intervals between normal heartbeats, recorded automatically by Apple Watch. Much training research uses RMSSD instead. Related, not interchangeable.

Shaffer and Ginsberg's overview of HRV metrics explains the difference best. The rule: compare your numbers with your own, never a chart's.

What is a normal HRV for me?

Your normal HRV is your own recent average and its usual spread, not a number on a chart. HRV differs widely between healthy people and bounces from day to day. The question is never whether 45 milliseconds is good, but whether it is unusual for you, beyond your normal noise.

That is why elite endurance research moved away from single readings. Plews and colleagues argued for appropriate averaging techniques. In their 77-day case comparison of two elite triathletes, the one who became non-functionally overreached showed a steady decline in the seven-day rolling average of ln rMSSD toward the race; the other's stayed stable. Single days were noisy for both. The trend carried the signal.

Buchheit's review of heart-rate monitoring adds: read any change against the measurement's own error and the smallest change that matters. A drop smaller than your usual wobble is not news.

Should I skip a run if my HRV is low?

Rarely. One low HRV reading alone is more likely noise, a late dinner or a poor night than a sign that training has gone wrong. A low reading beside a short night, a raised resting heart rate and heavy legs is more persuasive. Several low days in a row is the pattern to act on.

Even then, change the run; don't cancel it. Truly easy running adds little stress. The hard session on a morning when every signal points down is the one to move. Skipping every run on a low score turns a guide into an excuse.

Does HRV-guided training work?

Modestly, and mostly by helping people put hard sessions on better days. The controlled trials are small and short, and so are the gains over a sensible fixed plan. They consistently show that adjusting intensity to a morning reading is at least as good as ignoring it, sometimes with fewer hard sessions. Not magic. Not nothing.

Mind the limits: a few dozen people per study, mostly men, with careful morning recordings. Kiviniemi's trial used a simple rule: train hard while HRV held, go easy or rest when it dropped below its recent range or fell two days running. Vesterinen's runners trained hard only with HRV inside their smallest worthwhile change. The meta-analysis by Manresa-Rocamora and colleagues concludes that HRV guidance may be more effective for maintaining vagal HRV, with less likelihood of negative responses, but that any group-level advantage for fitness and performance is only by a small margin.

Is it OK to run after a bad night's sleep?

Usually, yes, for an easy run. A short night mostly changes how hard an effort feels and how well a demanding session goes, so it matters less for easy running than for a hard session. Sleep medicine societies advise adults to sleep seven or more hours a night on a regular basis.

A 2022 meta-analysis of acute sleep loss and physical performance found an average drop of about 7.6 percent, with significant effects across every exercise category, though tests the morning after the short night were largely unaffected. The hard session can wait a day.

Short sleep night after night is different: it deserves a lighter few days, not just a lighter morning.

What should I do on a low readiness day?

1. 01

   Check the pattern, not the number

   One input or several? One day or three? Do your legs agree?

2. 02

   Keep the run, lower the gear

   Swap Hard or Steady for Easy, at full-sentence pace.

3. 03

   Shorten it if you like

   A short easy run keeps the habit for little cost. Seven minutes is a real workout.

4. 04

   Move the hard session, don't stack it

   Put it on the next good day, never back to back with another hard run.

5. 05

   Rest when the signs point to illness

   Fever, feeling run down or pain that changes your stride: rest, and get it looked at if it persists.

When should I ignore my readiness score?

Ignore it when it has too little to go on, when you know why it is low, or when it says go and your body says stop.

• In the first week or two with a new device, before it has a baseline.
• After a one-off cause such as travel, a late night or a hot room: the number is right about the night, wrong about your fitness.
• On race morning. Nerves and travel change the night before. You trained for this; run it.
• When the score is high and you feel unwell. A good number never overrules a fever or a pain.

A high score is permission, not instruction. It doesn't make an unplanned hard run wise, or yesterday's hard session free.

How does Gears handle this?

Each morning Gears gives the day a gear. Today's gear reads "Gear 1 day", "Gear 2 day" or "Gear 3 day", from your sleep and your last few days, with the reason in plain words: "Short sleep", "Resting heart rate above your week", "Heart rate variability below your week", "Hard runs on two of the last three days" or "Nothing holding you back." On a Gear 1 day the hard run becomes an easy one. Disagree, and "Keep the plan" undoes it for today.

Beside it, a running readiness score from 0 to 100 (Optimal, Good, Fair or Pay attention) draws on heart rate variability, sleep, resting heart rate, training balance, sleep balance, yesterday's strain, body temperature, sleep regularity and breathing rate, each scored against your own last four weeks. Today's gear needs about a week of HRV and resting heart rate in Apple Health, and works on the watch alone, offline.

A logged fever, cough or fatigue, or a night when your body looks off its usual, makes the day easy or rest, in words like "Feeling run down. Keep it easy." One tap on Sick, Sore, Can't today or Make it easy moves your personalized training plan around you; nothing changes until you accept it. On the run, the pace target eases when your heart says the pace costs more than usual.

What it doesn't do: it can't feel your legs, doesn't diagnose anything and never names an illness. It reads what Apple Health holds; a night with no data is a blank. This is training guidance, not medical advice.