Breathing and oxygen
How you breathe and how much oxygen your blood is carrying. Measured mostly while you sleep, when breathing settles into its most regular pattern and a night-to-night comparison actually means something.
Which measurements you actually have depends on your devices and your lab results. Watches differ: an older model may never record heart-rate variability, while a newer one may not estimate VO₂max until it has recorded outdoor activity. A measurement you do not have is missing from your device, not missing from you.
Educational information — not medical advice or a diagnosis. Discuss your health with a physician.