In one sentence
Ten words you rate once a day, five for how worn down you feel and five for how lively, taken from a mood questionnaire that sport and sleep research has used for fifty years.
Why it matters
No sensor measures how you feel, and how you feel is not redundant with the sensors: a person whose overnight readings look recovered and who feels exhausted has a real finding, in both directions. The Profile of Mood States is the standard instrument for this; a shortened form that kept the items with the best face validity and internal consistency correlated above 0.95 with the full 65-item scale (Shacham 1983). VT asks only the two subscales that bear on recovery. Fatigue: worn out, listless, fatigued, exhausted, weary. Vigor: lively, active, energetic, alert, full of pep. Each is rated 0 (not at all) to 4 (extremely), for totals of 0 to 20 on each side.
What moves it
Sleep, most of all; then training load, being unwell, stress, alcohol, the time of day you answer, and mood, which the two subscales are designed to pick up. Fatigue and vigor are not mirror images: it is possible to be tired and lively on the same day, which is why both are asked.
How to answer it well
Each evening, quickly, about the day: the app asks to what extent you felt each word today; the words are deliberately plain so that there is nothing to work out. Answer it on good days too, or your usual will be built only from bad ones.
How Vitality Tracker uses it
Both totals feed the recovery side of Capacity, compared with your own usual: yesterday's fatigue and vigor answers are among the inputs to this morning's score, with the overnight readings and the night's duration and regularity. On the web the responses and the totals are stored separately, so the pattern of words is kept as well as the number; the mobile journal records the totals. Nothing else reads them.
How far to trust the number
Self-reported, which is what it is for. The subscales are reliable within a person when answered consistently; they are not comparable between people, and VT never compares them.
Sources
- Shacham S. A shortened version of the Profile of Mood States. J Pers Assess 1983;47:305–6. doi:10.1207/s15327752jpa4703_14
- Vitality Tracker, What VT measures (catalogue of 15 October 2026).