In one sentence
A 0-to-100 answer to one question, how much can you take on today, built from two sides, recovery and load, with the lower side holding the score down.
How it is built
The recovery side reads what the night and yesterday said about you: overnight HRV and overnight resting heart rate, how long you slept (naps included) and how regular the night was against your own pattern, and yesterday's fatigue, vigor, stress and body discomfort answers, each compared with your own usual rather than with anyone else's. Without an overnight reading there is no score. The load side reads what you have been asking of yourself: your logged training over the last seven days, or, if you have logged none for two weeks, your Activity Units. Whichever side is lower holds the score down, and VT records which one it was, so a low score always says why: not recovered, or already loaded. The recovery side is set in the morning; through the day the score falls as your training adds up, and VT keeps the morning value on record.
What it asks you first
Each morning, before any score appears, VT asks how you slept. That answer belongs to the Sleep Health Index, not to Capacity: Capacity is built from your readings and yesterday's answers, and how you feel this morning stays yours to weigh against it. The two can disagree, and both can be right: in 26 women tracked across a full menstrual cycle, overnight heart rate and temperature moved with the cycle while the women's own daily ratings of how ready they felt did not (Alzueta 2022).
What the evidence does and does not say
The inputs are well studied. Overnight heart rate and HRV respond to training, alcohol, the cycle and being unwell in consistent directions (Altini 2021), and HRV-guided decisions about training have a literature in sport, with the caution that the signal needs averaging and an individual baseline to be read at all (Plews 2013). The composite is not: no score of this kind, Capacity included, has been shown to relate to a health outcome or to say in advance how you will perform, and VT does not claim it. It is a morning summary of things that are each worth knowing, for you to weigh with how you feel.
How to use it
As a prompt, not an instruction. A low recovery-side score after a night you can explain is a reason to go easier, not a verdict; a low load-side score after three hard days is arithmetic. If the score and how you feel disagree for days on end, that is the thing to look at.
How Vitality Tracker uses it
Computed in the VT mobile app once the overnight readings and the night have synced, and again if a nap or a late-syncing night arrives. It feeds nothing else; Eir can explain the morning's score on request, with the inputs that set it.
How far to trust the number
Calculated, from measured, trend-grade and self-reported inputs; as good as the night's data and your answers. Its direction over a week on the same device means more than the number on any morning.
Sources
- Altini M, Plews D. What is behind changes in resting heart rate and heart rate variability? Sensors 2021;21:7932. doi:10.3390/s21237932
- Plews DJ, et al. Training adaptation and heart rate variability in elite endurance athletes. Sports Med 2013;43:773–81. doi:10.1007/s40279-013-0071-8
- Alzueta E, et al. Tracking sleep, temperature, heart rate, and daily symptoms across the menstrual cycle with the Oura ring in healthy women. Int J Womens Health 2022;14:491–503. doi:10.2147/IJWH.S341917
- Vitality Tracker, What VT measures (catalogue of 15 October 2026).