Table of Contents
How a watch measures the spo₂ on the wrist
A Health connected watch Do not aspire your blood (fortunately). It lights up the skin with LEDs (red ~ 660 Nm and infrared ~ 940 Nm, sometimes green in addition), then measures the light returned by the tissues. The PPG signal (photopleyysmography) captured by photododids varies to the rhythm of the pulse; The proportion of light absorbed by oxygenated/deoxygenated hemoglobin serves as an index to estimate the Spo₂. Estimate, not diagnose.
The PPG signal: reflection, not transmission
Unlike a Pulse oximeter finger (transmission through the fabric), the watch uses the reflection. The optical path is shorter, the infusion is lower, and everything moves: skin, hair, micro-transpiration, micro-movements. The algorithms isolate the pulsatile component (AC) of the stable component (DC) to reconstruct saturation. It’s subtle. Nothing is disturbing, nothing improves.
LEDs, photododids, software calibration
The precision is due to three things: quality of the LEDs and photodiodes, module geometry, and mathematical model. To limit noise: wave multi-long, intensity modulation, slippery average, movement filtering via accelerometer. Manufacturers calibrate their models with internal databases (variety of skins, temperatures, conditions). There are still structural limits, inevitable on the wrist.
Conditions for an exploitable measurement
Watches well positioned (one finger above the bone, firm contact without crushing), motionless wrist 30–60 S, clean and dry skin. Temperature: not too cold (vasoconstriction), not too hot. Dark tattoo? Disturbing. Dark skins? Algorithms are progressing, but variability still exists. As a result We repeat the measure at rest and read the trend, not the snapshot.
What is the SPO monitoring for daily monitoring
SPO₂ is not a trophy figure. She tells tissue oxygenation in your context. It is used to mark out sleep, monitor acclimatization at altitude, identify recovery drifts or episodes of poor ventilation. Not to decide on its own from a treatment. This is a false good idea.
Sleep: micro-falls and respiratory quality
At night, watches often sample at regular intervals. We observe the average sppo₂, the minimums and the episodes of deturation. A 92 % unique value when you wake up does not say anything. A rehearsal of night hollows coupled with alarm clock and a degraded breathing score? Interesting. We corrèle with heart rate, Respiratory Rate and micro-events.
For dedicated decryption, see connected watch and decrypted sleep apnea.
Sport and altitude: acclimatization without drama
In alpine or trail hiking at high altitude, the SPO₂ serves as a biological barometer. It drops when the partial oxygen pressure drops, this is normal. What matters: acclimatization kinetics, pace, symptoms. We avoid hasty interpretations in full effort; We measure at rest, we compare day J / D+1, we adjust sleep, hydration, intensity.
Real life: low signal, simple decisions
Long-haul trip, colds, stress, short night: spo₂ can wave. We don’t panic. We slow down, we rehydrate, we sleep. If the values remain abnormally low And Whether you have respiratory symptoms, you consult a professional. Point.
Reliability, limits, good practices
Convergent studies: good correlation at rest In healthy subjects, degradation with movement, vasoconstriction, humidity, certain phototypes, tattoos, too loose bracelets. Result: we relieve the measure rather than sacralized it.
Which distorts the measurement (and how to avoid it)
- Movements : Measure seated, motionless wrist, 30–60 s.
- Temperature : cold hands → rub a few seconds, restart.
- Position : a finger above the ulna, adjusted bracelet.
- Skin/inks : avoid tattooed/clean and dry zone.
- External lighting : Limit direct light under the sensor.
Interpret without over-react
We look at the tendency Multi-day, not the “score of the day”. We meet with sleep, training load, subjective sensation. A 95 % spo in a healthy subject at rest? Correct. 90 % isolated, running in the cold? Not significant. 88–90 % repeated at rest, breathless? There, we stop self-management and we speak medicine.
Clear reminder
A Oxygen saturation connected watch does not replace a medical oximeter or a professional opinion. Bulk : It is a room thermometer for your breathing, not a cabinet diagnosis.
Choose a Spo₂ connected watch according to your profile
We avoid endless technical sheets. We start from use: serious sleep, regular altitude, structured sport, or simple health curiosity. And we contrast.
For serious sleep
Favor: Continuous night samplingclear graphics (minimum, average, episodes), data export, correlation with respiratory frequency and micro-events. The watch must hold at night without activating superfluous screens. Simple, reliable, repeatable.
For altitude/hike
Find: easy rest measurement (quick access), altitude widgets, reliable barometer, supported negative temperatures, solid autonomy. We want a trend over several days of ascent, not just an anecdotal test at the top.
For regular sport
Useful: SPO₂ integration in recovery dashboards, drive load, heart rate variability (HRV). Respiratory health monitoring is not a metric of appearance, it is a layer of context. If it becomes stress, we cut the function. Yes, we have the right.
If the hesitation focuses mainly on the sport ecosystem, consult compare Garmin and Apple Watch for sport.
If you are very iOS (or very Android)
We favor consistency: clean notifications, clear health widgets, reliable synchronization over time. Integration into the health services of the smartphone has more than the “promise” marketing of the oxygen sensor. Always.
To view the Apple side generation deviations, see Apple Watch 11 vs Se 3 key differences.
Busy experience: what we check before clicking
We do not hunt the isolated “sppo₂” function. We buy a coherent whole. Real autonomy in full night (7–9 h with SPO₂ sampling), readability of graphs, software stability, simplicity of the health app. If possible: wrist test (housing shape, bracelet comfort), because a poorly tackled sensor makes everything else lapsed.
We read the update policy (frequency, corrections), we check the export of data (CSV/Healthkit/Google Fit), we spot the sampling frequency settings (continuous vs punctual). And we accept the principle: Better a “good enough” measure every day than a “perfect” measure never made. You check, you go back. Then you forget the sensor – you sleep. The goal is there.
Summary – the essentials to keep
- Definition : SPO₂ estimates oxygen saturation via PPG in reflection.
- Utility : sleep, altitude, recovery context. Not a diagnosis.
- Measurement quality : better at rest, motionless wrist, adjusted bracelet.
- Interpretation : Trend & context first, isolated value then.
- Boundaries : Movement, cold, tattoos, phototypes, poorly tackled sensor.
- Good practices : Repeat the measurements, correlate with sleep/FC/felt.
- Choice : prioritize the ecosystem, the night autonomy, the clarity of the graphs.
- Caution : In case of durably low symptoms or values → doctor.
