Why the body needs oxygen in the blood

Every breath brings in oxygen, and the blood carries it around the body — to the muscles, the brain, every cell. Blood oxygen saturation shows how well loaded with that oxygen your blood is right now.

In a healthy person at rest it is 95–100%. Those values are normal and nothing to think about. Below that is a reason to take a closer look rather than to worry straight away.

How the band measures it

tri has no screen and nothing needs switching on by hand. It simply shines an optical sensor through the skin of your wrist and estimates saturation from the way the light passes through the blood.

To save battery and stay out of your way, the band takes these readings at night — roughly every half hour. Across a night that adds up to many points, and together they show the overall picture rather than one random figure. You then see all of it in the app.

Worth understanding: this is an estimate, not a lab test. Accuracy depends on how the band sits and on what is happening to your arm.

How to read it and when to pay attention

A single low point is almost always interference, not a problem:

  • you turned over in your sleep and squashed your arm;
  • the band slipped or sits too loosely;
  • cold hands — in the cold, blood fills the wrist less well and the number dips.

You can safely let such one-off dips pass.

But when the values stay low night after night, that is worth listening to. If there are other worrying sensations alongside, see a doctor. Look at the trend across a week or two, not at a single night.

Blood oxygen is a background, a calm reference point. It does not tell you something is wrong with you. It simply helps you notice the moment when a closer look makes sense.

Sources

  • Night-time drops in oxygen saturation are a marker of sleep apnoea, which is widespread — Peppard P. et al., Am. J. Epidemiol., 2013. read
  • Time at night with saturation below 90% is linked to health outcomes — Henríquez-Beltrán M. et al., Sleep and Breathing, 2024. read

tri does not diagnose and does not replace a doctor. If something worries you, trust how you feel and see a specialist.