Field note
Overnight heart and recovery signals are not a clean bill of health, so a recovery app can say you are fit while you are sick in bed. The opposite mismatch also happens: you feel fine and the score still asks for rest.
A readiness score is a training call, not a health grade. It folds recent load, sleep you still owe, and overnight heart signals into one word: go hard, go easy, or rest. It is not the same question as how last night went.
Recovery, or how rested you look this morning, lives on one night. Readiness lives on a longer clock. It cares about the work you have been carrying, the sleep debt stacked over many nights, and whether yesterday's calm heart is sitting on a heavy week. Those can honestly disagree.
If you only remember one sentence: a green morning after a hard block is not automatically a day to push.
Banister and Calvert (1991) framed training as a balance of fitness built and fatigue spent. That is the oldest honest idea behind a readiness score. Recent work raises the fatigue side. Older accumulated work raises the fitness side. The score is a guess at which side is louder today.
| Thing people confuse | What it actually is |
|---|---|
| A health grade | A training call: hard, easy, or rest |
| Last night's recovery | A longer read of load, sleep debt, and overnight signals |
| A green morning after a hard week | Sometimes real freshness, sometimes a false-green trap |
Gabbett (2016) popularised comparing a short window of load to the base you have built. Spiking recent work above that base is the pattern coaches watch. Impellizzeri and colleagues (2020) then named the conceptual holes in treating any one ratio as an injury predictor. I am citing those papers as they sit in Aera's Field Guide source list. I have not opened the full texts for exact cut-offs here, so I am not attaching a magic ratio, a personal injury risk, or a "train at X percent" rule to your watch.
So if an app only praises a high score, ask what it weighed. Load without sleep debt is half a sentence. Overnight What is HRV? without the week behind it is the other half.
There is a trap that heart-based recovery reads walk into. After a hard block, heart-rate variability can surge above your usual while Why is my resting heart rate up? sits unusually low. That looks like peak recovery. It can be the opposite: the calming side of the nervous system slamming on the brakes.
Le Meur and colleagues (2013) described that pattern in functionally overreached athletes: parasympathetic hyperactivity that looks like super-recovery on a heart chart. Push hard on one of those mornings and you can dig a hole that takes days to climb out of. The honest read treats a very green overnight pair sitting on a heavy recent week with suspicion, not celebration.
High variability can still mean real adaptation. No wrist signal can always tell those two apart. That is why a readiness score that only watches last night is easy to fool, and why recent load belongs in the same sentence.
A consumer score can notice that recent work is high, that sleep has been short for a stretch, or that overnight heart signals look tight. It cannot see soreness, a niggle in a knee, an argument, or an illness before it reaches the wrist. Treat the word as a starting opinion your body gets to overrule.
One night is noisy. One workout is noisier. The useful move is the trend: several days asking for easy, or a score that stays high while you feel wrecked. Believe the wrecked feeling first.
Different apps will disagree on the same morning. They weigh load, sleep, and the heart differently. Disagreement is not proof that one of them secretly knows the truth.
A readiness score is a training call from recent load, sleep debt, and overnight signals. It is not a health grade, and it is not the same as one night of recovery. A green morning after a hard block is not automatically a day to push.
I write these because I build Aera, an iPhone app that reads Apple Health against your own baseline and puts the research and the error bars underneath every number it shows you, including this one. You do not need it to use anything above.