In one sentence
Your glucose first thing in the morning after at least eight hours without food, which is the oldest glucose test and the one that says how your liver and your insulin get on when there is nothing to digest.
Why it matters
Overnight, with no food coming in, your glucose is whatever your liver releases minus whatever your tissues take up, and the balance is set by insulin. A fasting value is therefore a reading of the resting state of that system. It is also a diagnostic test: the standards set 7.0 mmol/L (126 mg/dL) as the fasting cut-point for diabetes and 5.6 to 6.9 as the band the ADA calls impaired fasting glucose (ADA 2024), and most laboratories print a reference range whose top sits near 5.6. Those are the laboratory's and your doctor's figures; The app shades its own guide band of 3.9 to 5.5 mmol/L on the fasting-glucose chart, amber outside it, and marks readings below 3.0 or above 13.9 in red. With a fasting insulin from the same draw it gives HOMA-IR, the home estimate of insulin resistance (see the bloodwork entries).
What moves it
It is the slowest glucose number to move, and the one most often misread. A late or large dinner raised overnight glucose in healthy volunteers (Gu 2020), and the morning value carries some of it. The dawn effect is real: many people's glucose climbs in the hour before waking as the body prepares for the day, so a high reading on waking is not the same as a bad night, and a sensor shows the climb. Alcohol the evening before tends to lower it. Being unwell raises it. A single short night did not change fasting glucose in Donga's nine healthy adults even as it cut their insulin sensitivity by a quarter (Donga 2010), which is the point: the fasting number can look fine while the day's responses have changed, and the sensor sees what the fasting value cannot. Day to day it wobbles by a few tenths for no reason you will find.
How to measure it well
By laboratory, at the baseline panel, after a true overnight fast of 8 to 12 hours with water only. By sensor, VT takes the reading closest to the moment you stopped sleeping, within half an hour, from your sleep record; a finger-prick you log as fasting within half an hour of waking takes its place. By finger-prick, in the morning before anything, with clean dry hands, and the same meter each time. Keep the three apart; they are three instruments.
How Vitality Tracker uses it
The day's fasting glucose carries 30 percent of the glucose area of the Vitality Score, scored on the period's mornings, or, if there are none, the last week's, or a single morning up to 14 days old. VT keeps one fasting value per day: from a sensor, the reading at the moment your sleep ended, not when you got up, unless a finger-prick near waking takes its place; it does not carry forward values from a sick day or the three days after it.
How far to trust the number
Laboratory: precise, and the reference for the others. Sensor: the morning's tissue-fluid reading, within the sensor's usual error and subject to the night's compression artifacts, so read the week's mornings rather than one. Meter: a finger-prick's precision, a few tenths either way.
Sources
- American Diabetes Association. 2. Diagnosis and classification of diabetes: Standards of Care in Diabetes 2024. Diabetes Care 2024;47(Suppl 1):S20–42. doi:10.2337/dc24-S002
- Gu C, et al. J Clin Endocrinol Metab 2020;105:2789–802. doi:10.1210/clinem/dgaa354
- Donga E, et al. J Clin Endocrinol Metab 2010;95:2963–8. doi:10.1210/jc.2009-2430
- Vitality Tracker, What VT measures (catalogue of 15 October 2026).