# Overnight blood oxygen (SpO₂) The share of hemoglobin carrying oxygen; 95 to 100 at sea level. Wearable values carry 2 to 3 points of error; read as a trend. Type: biomarker · Unit: % · Domain: Cardio-Respiratory Health · Accuracy: Measured · Importance: Core Also called: oxygen saturation, blood oxygen saturation, pulse ox, overnight SpO2, oxygen level ## In one sentence The share of the hemoglobin in your blood that is carrying oxygen, read through the skin by the red and infrared light of your ring or band while you sleep. ## Why it matters Oxygen saturation is the one overnight reading that says something about the lungs and the airway rather than the heart and the nervous system. In a healthy adult at sea level it sits in the high nineties all night and barely moves, so a night that averages lower than your own usual, or a pattern of repeated dips, is information. In the pooled laboratory records of 5,273 healthy adults, mean overnight saturation fell by 0.6 points and the night's minimum by 1.8 points for each decade of age (Boulos 2019), which is the scale of change worth noticing. ## What moves it Downward: altitude, most of all; a chest infection or a blocked nose; sleep-disordered breathing, which shows as dips rather than a lower average; alcohol or a sedative near bedtime; a cold finger, which is a measurement problem rather than a body one. Upward: nothing you can do beyond breathing room air at sea level, which is why Symptom Radar watches this reading downward only. ## How to measure it well A snug ring or band on a warm finger or wrist, and the same device every night. Optical oxygen readings fail quietly when the finger is cold or the ring loose, and a device that could not get a clean reading reports nothing, which VT shows as a dash rather than a guess. ## How Vitality Tracker uses it A main reading in Symptom Radar (weight 1.0, lower is the direction it watches), compared with your own 30-day baseline. Symptom Radar does not yet start a new baseline for this reading by itself when you change device, so expect a few unsettled nights after a switch while the baseline catches up. Two rules sit outside the baseline. A night below 94 percent can turn the day's result red on its own, whatever else was measured; and once VT has a week of your oxygen readings, a night below 90 percent also adds a safety note to the result, whatever your usual. Capacity does not use it. ## How far to trust the number Measured, near the top of the range where almost every night of a healthy adult sits: in 100 outpatients with lung disease, an Apple Watch's saturation correlated 0.81 with two clinical pulse oximeters, its heart rate 0.995, and in that study skin color, wrist hair and nail enamel made no difference (Pipek 2021). Agreement between any optical sensor and the laboratory weakens as saturation falls, so a low night is a reason to look rather than a figure to quote. ## Method Pulse oximetry: the ratio of red to infrared light absorbed by pulsing blood, calibrated by the maker to saturation. Stored as the night's average; baseline and comparison as for the other main readings, with the 94 and 90 percent rules applied whatever the baseline says. ## Sources 1. Boulos MI, Jairam T, Kendzerska T, Im J, Mekhael A, Murray BJ. Normal polysomnography parameters in healthy adults: a systematic review and meta-analysis. Lancet Respir Med 2019;7:533–43. [doi:10.1016/S2213-2600(19)30057-8]() 2. Pipek LZ, Nascimento RFV, Acencio MMP, Teixeira LR. Comparison of SpO₂ and heart rate values on Apple Watch and conventional commercial oximeters devices in patients with lung disease. Sci Rep 2021;11:18901. [doi:10.1038/s41598-021-98453-3](https://doi.org/10.1038/s41598-021-98453-3) 3. Vitality Tracker, How it works: Symptom Radar. ## Related - [Overnight respiratory rate](https://vitalitytracker.app/knowledge/overnight-respiratory-rate) - [Overnight resting heart rate](https://vitalitytracker.app/knowledge/overnight-resting-heart-rate) - [Symptom Radar and Symptom Trend](https://vitalitytracker.app/knowledge/symptom-radar-and-symptom-trend) - [Sleep Health Index](https://vitalitytracker.app/knowledge/sleep-health-index) Reviewed October 2026 · v1 Source: https://vitalitytracker.app/knowledge/overnight-blood-oxygen