Blood oxygen, or SpO2 (peripheral oxygen saturation), is the percentage of hemoglobin in your red blood cells that's carrying oxygen. For healthy people at sea level it usually sits between about 95% and 100%, and it stays remarkably steady from night to night. Because it's so stable, a dip below your usual overnight range is worth noticing.
How Blood Oxygen Is Measured
Your lungs load oxygen onto hemoglobin, and your blood carries it to your muscles and organs. SpO2 tells you how saturated that hemoglobin is. Wearables estimate it optically, by shining red and infrared light into the skin at your wrist and measuring how much reflects back. Oxygen-rich and oxygen-poor blood absorb that light differently, the same principle a fingertip pulse oximeter uses.
Wrist readings are best for spotting trends rather than for precise single measurements. Fit, skin temperature, movement, and how you're lying can all affect a reading, so a single low number isn't necessarily meaningful.
What can lower your overnight blood oxygen:
- High altitude, where there's less oxygen in the air
- Respiratory illness, or congestion that disrupts your breathing during sleep
- Breathing disruptions during sleep
- A loose or poorly positioned watch, which can cause inaccurate readings
How Blood Oxygen Relates to Training
For most healthy athletes at sea level, blood oxygen stays in the high 90s and doesn't change much with training. It becomes most relevant in a few situations: training at altitude, where readings drop while your body acclimatizes; during illness; and as a sanity check when other recovery signals look off.
If your readings are often well below your usual range, or you have symptoms like shortness of breath, loud snoring, or daytime sleepiness, talk with a doctor. A wearable can't diagnose a sleep or breathing condition.
How FITIV Uses Blood Oxygen
FITIV Pulse reads blood oxygen from Apple Health, recorded while you sleep by your Apple Watch or another device that writes it to Apple Health. Apple Watch Blood Oxygen availability depends on your watch model and where it was purchased (Apple).
- Vitals card: your latest overnight SpO2 appears on the Today screen as a percentage, labeled Typical, Above, Below, Well Above, or Well Below compared with your own typical range.
- Vitals Summary: see your typical range, a 30-day chart, and your 7-day and 30-day averages.
- Recovery: blood oxygen is one of the overnight vitals behind your Recovery score. Stable, typical values support recovery; a dip below your usual range holds it back.
You can choose whether FITIV uses your blood oxygen from sleep (the default) or from the whole day in Settings → Today Tab. Pair it with your respiratory rate and HRV for the fuller picture.
FITIV's vitals are for fitness tracking and general wellness, not for diagnosing or monitoring medical conditions. If you have health concerns or unusual readings, talk with a healthcare professional.
Frequently Asked Questions
What is a normal blood oxygen level?
For healthy people at sea level, blood oxygen is usually between about 95% and 100%. It can read lower at altitude. If your readings are consistently low or you feel unwell, talk with a doctor.
Does blood oxygen affect my Recovery score in FITIV?
Yes. Stable, typical blood oxygen supports your Recovery score, and a dip below your usual overnight range holds it back.
Why does my Apple Watch blood oxygen reading vary?
Wrist-based readings are affected by fit, movement, skin temperature, and sleeping position. That's why FITIV compares each night with your own typical range and shows trends, rather than treating one reading as definitive.
