Sleep, HRV and readiness: what your morning numbers mean

Wearables now hand you a stack of numbers every morning: hours slept, resting heart rate, heart rate variability, sometimes a single readiness score. It's tempting to let them decide your day. Used well, they're a helpful signal. Used badly, they make you anxious about one restless night. The difference comes down to one idea: trends matter, single readings mostly don't.

Heart rate variability, in plain terms

Your heart doesn't beat like a metronome. The gap between beats varies slightly, from one beat to the next. Heart rate variability (HRV) measures that variation, usually in milliseconds.

The variation is shaped largely by your autonomic nervous system. When you're rested and relaxed, the branch that slows the heart tends to have more influence, and HRV is often higher. Hard training, poor sleep, illness, alcohol and stress can all push it lower.

Three things are worth knowing before you read your own HRV:

• It's personal. Healthy people have very different normal values. Comparing your number with a friend's tells you nothing. Compare it with your own baseline.
• It's noisy. Day-to-day swings are large, even when nothing has changed.
• Direction needs context. Lower usually means more strain, but not always.

That last point comes from research on elite endurance athletes. A review in Sports Medicine by Plews and colleagues found that in elite athletes, both increases and decreases in HRV have been linked to poor adaptation, and that fitness gains sometimes came with lower HRV. The authors recommended using appropriate averaging techniques rather than reacting to day-to-day values, and long-term monitoring to learn each athlete's individual pattern.

Resting heart rate

Resting heart rate is simpler. A sustained rise of a few beats per minute over your normal, especially alongside lower HRV, often points to accumulated fatigue, poor sleep or an illness coming on. A lower resting heart rate over months often reflects better aerobic fitness.

Like HRV, it varies from night to night with alcohol, a late meal, a warm room or a stressful day. One high reading is just one reading.

Sleep: duration and consistency

Sleep is the input with the clearest link to how training feels. The CDC recommends seven or more hours a night for adults aged 18 to 60, and seven to nine hours for those aged 61 to 64.

Two things matter beyond the total:

• Consistency. Going to bed and waking at similar times helps your body clock and makes each night's sleep more restorative.
• Debt over several nights. One short night is usually manageable. Several in a row add up, and that's when hard sessions start to feel much harder than they should.

Wearable sleep stages are estimates from movement and heart rate, not a sleep lab. Total sleep time and bedtime regularity are more trustworthy than the breakdown into stages.

How readiness combines the signals

A readiness score is an attempt to roll sleep, HRV and resting heart rate into one view of how prepared you are to train. The good versions share a few properties:

1. They compare you with yourself. Your baseline, not a population average.
2. They smooth over several days. A rolling view filters out one-off nights.
3. They look for agreement. Short sleep plus low HRV plus raised resting heart rate is a much clearer signal than any one of them alone.

Why one bad night matters less than you think

Imagine three mornings. Monday: good sleep, normal HRV. Tuesday: four and a half hours after a late flight, HRV well down. Wednesday: back to normal.

If you plan from single days, Tuesday looks like a red flag. If you plan from a short trend, Tuesday barely moves the picture. The second approach is usually right. One short night rarely changes what you can do; it mostly changes how you feel doing it.

What should get your attention is a run of days pointing the same way. Three or more mornings of short sleep, lower HRV and a raised resting heart rate are worth acting on. The IOC's consensus statement on load and illness lists lack of sleep among the stressors that can raise illness risk, alongside rapid jumps in training load. Those are the weeks to ease off.

What to do with your numbers

Here's a practical approach:

• Trend normal: train as planned.
• Trend dipping for a few days: keep the session, but swap high intensity for easy work. If you lift, keep the main lift and drop the extras.
• Trend low and you feel unwell: rest, and treat it as possible illness. Training when you're sick, travelling or short on sleep covers what to do next.
• Numbers and feelings disagree: your own sense counts. Start the session; if the first ten minutes feel heavy, make it easy.

This is the logic behind an adaptive training plan: recovery data nudges the week, it doesn't run it. For lifters, especially older ones, it's also a good guide for when to hold weights steady; strength training after 50 uses the same idea.

How Stwong reads your mornings

Stwong reads your sleep and recovery from Apple Health each morning. Readiness is treated as a trend, the median of the last three days, so one bad night alone doesn't change the plan. When the trend dips, your coach proposes a change, such as an easier session or a moved one, which you can apply, tweak or keep. The morning brief tells you today's session, how you slept and one thing worth doing. Raw Health data stays on your iPhone.

Nova, the performance analyst, is for people who want the numbers first. Precise, compact, and never inventing a number your data doesn't hold.

Sleep short two nights, load on plan. Today goes easy; protect Saturday's long session. One reading is a data point; a week is a trend.

If you'd like your morning numbers turned into one clear decision, take a look at Stwong.