Blood Oxygen (SpO2)

What SpO2 Measures

Hemoglobin is the protein in your red blood cells that carries oxygen from your lungs out to the rest of your body. SpO2 is the share of that hemoglobin currently loaded with oxygen, expressed as a percentage. Oxygenated and deoxygenated hemoglobin absorb light differently: oxygenated hemoglobin lets more red light through and absorbs more infrared, while deoxygenated hemoglobin does the opposite.^[1] A pulse oximeter, whether it's a clip-on hospital sensor or a wearable, measures that difference and calculates a saturation number from it.

How It's Measured

Consumer wearables use the same light-absorption principle as a clinical pulse oximeter, but read it through the skin at the wrist instead of a fingertip. An LED shines red and infrared light into the wrist, a photodetector picks up what bounces back, and the device's algorithm converts that pattern into an SpO2 estimate.^[1] Movement, a loose band, and stray light all degrade the signal, so most wearables sample it overnight while the wrist stays still. A 2024 study comparing a commercial smartwatch's overnight SpO2 readings against a lab-grade oximeter during sleep studies found the watch tracked the reference closely, with error low enough to meet the accuracy bar set for reflectance-type oximetry sensors.^[2]

Why People Track SpO2

Most wearables that track HRV or skin temperature overnight report SpO2 in the same sleep summary, so it shows up automatically once you own one, not because people go looking for it. The honest picture is that unlike several other overnight biometrics, SpO2 doesn't move in a consistent pattern across the menstrual cycle. A 2024 study following women through three cycle phases found resting SpO2 held steady from the early follicular phase through the mid-luteal phase, with no significant difference between them.^[3]

That's a real contrast with metrics that do shift by phase. Wrist skin temperature rises in a fairly predictable pattern after ovulation, which is why wearables use it to estimate when ovulation likely happened.^[4] Resting heart rate and HRV move too, with heart rate generally running higher and HRV lower in the luteal phase.^[5] See HRV and Resting Heart Rate Across Your Cycle for more on that pattern. SpO2 sitting apart from that group is itself a useful thing to know: it's a metric your wearable tracks that isn't a cycle signal.

How Go Go Gaia Tracks SpO2

Go Go Gaia pulls SpO2 automatically from Apple HealthKit, so any overnight reading your Apple Watch or connected wearable already records syncs into the app without extra logging. The app charts it alongside your cycle phase and other biometrics like HRV, resting heart rate, and wrist temperature, so you can see the full set of overnight signals together instead of checking each one in a separate app.

What the Research Shows

The clearest study on this compared resting SpO2 across the early follicular, late follicular, and mid-luteal phases in a group of eumenorrheic women and found no significant difference between phases, matching what earlier resting-state measurements had shown.^[3] That's a null result, not a gap in the research: the study was specifically designed to detect a cycle-phase effect and didn't find one.

For comparison, the metrics that do shift across the cycle have their own dedicated research base. Wrist skin temperature tracks the postovulatory rise in progesterone closely enough that wearables use it to estimate ovulation after the fact.^[4] Vagally-mediated HRV drops in the luteal phase in step with rising progesterone.^[5] SpO2 not following either pattern is the useful finding here, not an absence of data.

Related Terms

Frequently Asked Questions

Educational information based on published sources. Not medical advice. For personal concerns, please consult your doctor.

Does blood oxygen change across your menstrual cycle?

Research hasn't found a consistent pattern. A 2024 study tracking women through the early follicular, late follicular, and mid-luteal phases found resting SpO2 didn't vary between them. That's different from metrics like temperature, resting heart rate, and HRV, which do shift across the cycle.

Why does my wearable measure SpO2 overnight instead of during the day?

Movement, ambient light, and a loose fit all interfere with the light-based sensor wearables use. Sleep gives the sensor a still wrist and consistent contact for longer, which produces a cleaner reading than a quick daytime check.

Is SpO2 the same thing as heart rate variability or resting heart rate?

No. HRV and resting heart rate come from the electrical timing of your heartbeat and reflect autonomic nervous system activity, which shifts across the cycle. SpO2 comes from a separate optical sensor and measures oxygen carried in the blood, a different physiological signal that doesn't follow the same cycle pattern.