Blood oxygen is the one overnight metric where NOYU breaks its own favourite rule. For almost everything else, what matters is your number against your own history. Here, an external range applies.
What blood oxygen measures
Blood oxygen, or SpO2, is the percentage of the haemoglobin in your blood that's currently carrying oxygen. It answers a simple question: how well is oxygen getting from your lungs into your bloodstream?
For healthy adults the answer is "very well, almost all the time". Normal saturation sits between 95 and 100 percent, and it stays there through sleep, stress, exercise and everything else an ordinary week throws at you. That's what makes the metric different from HRV or heart rate: it isn't supposed to move.
Where the number comes from
Wearables estimate SpO2 by shining red and infrared light into your skin and measuring how much of each is absorbed. Oxygen-rich and oxygen-poor blood absorb these wavelengths differently, so the ratio reveals the saturation.
When NOYU shows blood oxygen, it's your overnight average: readings your device took while you slept, averaged across the night. NOYU passes this number through from your device as reported. It doesn't recalculate it or adjust it.
Not every device measures it, and some only measure it when a setting is switched on. If yours doesn't report blood oxygen, the metric won't appear, and nothing else in NOYU is affected.
Why it's read against a range, not your baseline
Most overnight metrics vary widely between healthy people - your HRV can be a third of your training partner's while both of you are in perfect health - which is why NOYU reads them against your own baseline.
Blood oxygen is the opposite case. Healthy overnight saturation occupies a narrow band that's the same for essentially everyone, so an absolute range carries real information here in a way it doesn't for HRV.
How NOYU uses it
Blood oxygen appears in your data with its trend across recent nights, like your other metrics.
It also makes a small contribution to Readiness when your device reports it: full marks for a healthy overnight average, falling as the average drops below that band. It's the smallest part of the score. When your device doesn't report it, NOYU drops it and rebalances across the metrics you do have, so its absence never costs you.
One overnight average can't show brief dips or tell you why a night read low - and a wrist estimate comes with real caveats, which the companion article covers.
