How Sleep and Recovery Are Tracked

The device

Sleep and recovery figures come from a Hume Health Band worn overnight. The band reports sleep quality, heart rate variability, resting heart rate, active calories, SpO2 and a derived recovery score. Metabolic capacity is a score the device calculates rather than a measurement, and it is treated as such.

Why these are worth tracking

Sleep is the dimension most often reported to change during fasting and the one most easily dismissed as imagination. Overnight HRV and resting heart rate give a continuous, unglamorous record that either supports what the written notes say about a fasting night or contradicts it. Both outcomes are worth publishing.

What a wearable actually measures

The band measures pulse at the wrist optically and movement with an accelerometer. Everything else — sleep stages, recovery, metabolic capacity — is an algorithm's interpretation of those two signals. That means:

• Sleep-stage figures are estimates, not the polysomnography a sleep laboratory would run
• Scores are not comparable with those from a different brand of device
• A poorly seated band produces plausible-looking nonsense
• Changing device or firmware can shift the series without anything changing in the body

The figures are published because the trend within one device on one wrist is informative. They are not published as clinical measurements.

What this measurement cannot tell you

A single reading is close to meaningless on its own. What matters is the shape of the series across a year, read alongside the fasting schedule, sleep, training and the notes for that week. Nothing on this site is presented as evidence that fasting caused a change: this is one person, with no control condition, and many things change over a year besides the fasting days.