Shindo The short answer#
Because sleep is one input, not the score. Every serious readiness model also reads heart-rate variability, resting heart rate and the training load you are carrying — and any of those three can drag the number down on a night you slept eight solid hours.
That is usually the useful case, not the broken one. The mornings when you feel fine and the number disagrees are precisely the mornings the number is telling you something you could not feel.
Here are the eight causes, roughly in order of how often they are the answer.
1. You are carrying training fatigue#
The most common explanation by a wide margin. Recovery from a hard session takes 24–72 hours, and the second day is often worse than the first. If yesterday was a long ride or a hard interval session, your body is still repairing, and no amount of sleep short-circuits that.
How to tell: look at your form, or fitness-minus-fatigue number. If it is deeply negative, the score is doing its job. This is not a problem to be solved — it is the cost of the training working.
2. You drank alcohol#
Alcohol is the single most reliable way to suppress overnight heart-rate variability. Two glasses of wine will measurably reduce it, even when total sleep time is unaffected and you feel fine. It elevates heart rate, blocks the deepest sleep, and the effect is strongest in the first half of the night.
How to tell: you know. If HRV is well down and resting heart rate is up several beats after a social evening, that is what you are looking at. It clears in a day or two.
3. You ate late#
A large meal within about three hours of sleeping keeps digestion running through the early night, which raises heart rate and suppresses HRV. The effect is real and frequently mistaken for something more serious.
How to tell: the pattern repeats on late-dinner nights and is absent otherwise.
4. The room was too warm#
Thermoregulation during sleep has a large effect on both heart rate and HRV. A hot room, too many layers, or a warm night in summer produces a suppressed reading without any subjective sense of having slept badly.
How to tell: it correlates with the weather, and with which room you slept in.
5. You are getting ill#
Immune activation raises resting heart rate and suppresses HRV before you feel symptoms — often by 24 to 48 hours. This is one of the genuinely valuable things a readiness score does, and one of the few places it can be more informed than you are.
How to tell: resting heart rate elevated several beats, HRV well down, and no obvious behavioural explanation. If it holds for two or three days with no training reason, treat it seriously and back off. This is not paranoia — the cost of an easy day you did not need is trivial next to the cost of training through the start of an infection.
6. You travelled, especially by air#
Flying, time-zone change, dehydration and disturbed sleep all suppress recovery markers for a few days. Altitude does too, and more strongly than most people expect.
7. Life stress, not training stress#
The autonomic nervous system does not distinguish between a hard interval session and a difficult week at work. Psychological stress produces genuine physiological load and shows up in exactly the same markers.
How to tell: training load is unremarkable, no illness signs, and you know what is going on in your life. The score is right — the fatigue is real — it simply is not training fatigue, and the response is still to take the day easier.
8. It is a measurement artefact#
The least interesting explanation and worth eliminating before acting. A loose strap, a watch worn differently, a device that sampled a different part of the night, an unusually short sleep window that gave the algorithm little to work with.
How to tell: the reading is wildly out of line with the previous week and everything else looks normal. One anomalous day among otherwise consistent ones is more likely to be noise than signal — which is why trends matter more than any single morning.
How to work through it#
- Look at the drivers, not the score. Any app worth using shows which inputs pulled the number down. A low score driven by form is a different situation from one driven by resting heart rate.
- Check resting heart rate specifically. Elevated resting heart rate plus suppressed HRV is the illness pattern. Suppressed HRV with normal resting heart rate is more often behavioural — alcohol, heat, a late meal.
- Ask what changed yesterday. Most low mornings have an obvious cause once you look.
- Count the days. One low day is noise. Three consecutive low days with no clear explanation is a signal.
- Include yourself in the decision. If you feel genuinely good and there is a plausible explanation the score cannot see, train. If you feel bad, take the easier option regardless of what the number says.
The mistake to avoid#
Do not treat a single low reading as a command to skip training. These scores are noisy at the single-day level, and an athlete who takes a rest day every time an app shows yellow will not train enough to improve.
Their real value is in the trend: a slow slide over one to two weeks is the pattern that precedes non-functional overreaching, and it is close to invisible day to day. That is what to watch.
Shindo always shows the four drivers behind readiness — sleep, heart-rate variability, resting heart rate and current form — so a low score arrives with its reason attached. Where an input is missing it says so rather than substituting a value. Read how it is computed.
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