Blood Oxygen on Apple Watch: Reading Overnight SpO₂ Calmly

Spotting 93% on a morning blood oxygen chart can be unsettling, especially if you have heard that 95% is the usual line. A single low reading can say as much about the measurement as about you. This guide covers what the Blood Oxygen app measures, why readings vary, and how My Bio Age summarises a night.

What the Blood Oxygen app measures

Blood oxygen, or SpO₂, is an estimate of how much oxygen your blood is carrying compared with the most it could carry, shown as a percentage, as the American Thoracic Society explains. According to Apple's Blood Oxygen app guide, the app runs on Apple Watch Series 6 or later and every Apple Watch Ultra, but not on Apple Watch SE. As well as on-demand checks, it takes occasional background readings, usually while you are still, and measures during sleep only when Track Sleep with Apple Watch is on.

Availability varies. By default, the app is offered only in certain countries and regions.
Some US models work differently. On watches bought in the US on or after 18 January 2024 with part numbers ending in LW/A or LS/A, readings are analysed on iPhone and the results appear in the Health app.
Adults only. The app is designed for people aged 18 and over.
Wellness, not medicine. Apple states that the measurements are not intended for medical use, including self-diagnosis.

Readings are saved to the Health app, and Apple's HealthKit documentation describes each sample as a percentage of oxygen in the bloodstream, stored as an individual value rather than a running total.

Why readings vary

Even in ideal conditions, Apple says, the watch may not always get a reliable reading. Factors include:

Skin perfusion. How much blood flows through your skin differs between people and with your surroundings; in the cold, it may be too low at the wrist for a reading.
Tattoos. Some tattoo inks and patterns can block the sensor's light.
Movement and posture. Motion can stop both background and on-demand readings, and so can some positions, such as letting your arm hang by your side or making a fist.
Fit. The band should be snug but comfortable, with the back of the watch touching your wrist.

MedlinePlus describes clip-on pulse oximeters, usually worn on a fingertip, which pass light through the skin and estimate oxygen from what a sensor picks up. It lists cold skin, poor circulation and darker skin pigmentation among things that can affect accuracy, notes that their results can be 2% to 4% above or below your actual level, and gives 95% to 100% as normal. That range is for medical pulse oximeters; a watch shines its light against the wrist and is a wellness device, so treat its readings as estimates too. Apple adds that some people live normally below 95% and that slightly lower values during sleep are expected.

How My Bio Age summarises a night

One value per night. My Bio Age groups readings that start within the same 9 pm to 9 pm window and labels the night by the day you wake up. The night's value is the median, so one unusual reading moves it less than it would an average. Because the window runs until 9 pm on the day you wake, readings from that day count towards the night too, including on-demand checks and readings other apps or devices save to Health. So the SpO₂ (Night) value is not purely overnight, and even a single daytime check can count as a recorded night.
The share below 95%. The app also records what proportion of each night's readings fell below 95%. It feeds the calculations below rather than appearing as a number of its own.
A weekly average. For each complete Monday-to-Sunday week, the app averages the nightly values. Five or more recorded nights give full confidence; fewer count for less.
A card in Overview. The SpO₂ (Night) card shows your latest complete week's average and the change from the week before.

Where it is used

Context for the weekly MyBioAge Index™. SpO₂ is one of the 12 core Index inputs, with one of the smallest weights, so read it as context rather than a number to nudge.
Training Load's Oxygen Health signal. In Training Load, Oxygen Health reflects how often your readings fell below 95% across the nights recorded in the last seven days. With no readings in that window it reads as low, so check for missing nights first.
A data checklist item. When the Index lists the data it still needs, Night SpO₂ can appear with the number of nights missing. It looks for about three recorded nights a week, or six in the past 14 days. Missing SpO₂ nights will not, on their own, hold the Index back as Limited, but they do count against its data coverage.

When to talk to a clinician

The SpO₂ detail view notes that healthy night-time values usually sit at 95% or above, and that frequent drops below 92% should be discussed with a clinician. That concerns a pattern across nights, not one reading. Note how often it happens and anything that changed, such as illness, higher elevation or a looser strap, and take that to a GP or another qualified clinician.

Symptoms matter more than any number. The NHS advises calling emergency services straight away for severe difficulty breathing, a tight or heavy chest, lips or skin turning very pale, blue or grey, or sudden confusion. Don't wait to check your watch.

Blood oxygen readings, and My Bio Age's nightly summary of them, are not a sleep apnea test, and normal-looking nights do not rule anything out. Apple's separate sleep apnea notifications, available on some newer models, use the watch's motion sensor to look for breathing disturbances, and even those can miss some people who have the condition. The NHLBI lists signs such as loud snoring, gasping for air and breathing that stops and starts, and notes that a sleep study may be needed for a diagnosis. If you or a partner notice them, speak to a healthcare provider, whatever your readings show.

Putting it together

Overnight blood oxygen is one strand of the wider picture of your nights. Read it alongside sleeping respiratory rate, wrist temperature and our sleep duration guide. If nights are missing, start with the Apple Health data-quality checklist.

If you already have blood oxygen readings in Apple Health, My Bio Age brings those summaries together with sleep and other nightly signals. Download My Bio Age free on the App Store. The iPhone app is completely free, with no in-app purchases, subscription or account required, and it processes your health data privately on your iPhone.

My Bio Age processes the health data it uses on your iPhone and provides general wellness information, not medical advice. It does not diagnose breathing, heart or sleep conditions, and a reading is never a reason to change your treatment on your own.