It means that Fitbit has beaten Apple to having this health feature, as it doesn’t yet work with the Apple Watch. Read more on the battle of Fitbit vs Apple Watch.

Oxygen saturation can be estimated with a device called a pulse oximeter that has an Sp02 sensor, which shines red and infrared light onto your skin and blood vessels, and uses the light bouncing back to see how much oxygen is in your blood. This is usually a device that is clipped onto your finger.

Fitbit uses its trackers’ heart-rate measuring red and infrared lights. It’s not as accurate as the medical device as there are fewer blood vessels on the wrist than in the finger. To be more accurate you need to measure the light across a cross-section of a vein.

While it can’t measure the blood’s exact oxygen saturation level, it can detect major changes.

Apple’s Watch has no Sp02 sensor capabilities, although there is a third-party iPhone app called Pulse Oximeter that uses the iPhone’s camera to detect pulse and oxygen levels from a fingertip. However, the app hasn’t got great reviews from medics.

Richly oxygenated blood is red and reflects more red light than infrared light. Poorly oxygenated blood, on the other hand, is blueish red, and reflects more infrared light than red light.

Fitbit Estimated Oxygen Variation graph

The graph can be found in the mobile Fitbit app under the Sleep tile. It works with all Fitbit smartwatches and the Charge 3 tracker. Read our Fitbit Charge 3 review, Fitbit Versa Lite review, Fitbit Versa 2 review and Fitbit Ionic review.

It approximates the changes in your blood oxygen saturation while you sleep, and can be used to check if you might be suffering for breathing issues such as sleep apnea.

The heart pumps blood to send oxygen around the body.

Blood oxygen saturation measures the percentage of your blood that’s saturated with oxygen. Typically, it’s at 95-100%, meaning the blood is carrying as much oxygen as it can.

However, if you stop breathing or have other lung issues, the oxygen saturation level can fall as less oxygen enters the body.

Blood oxygen saturation normally fluctuates, but big variations can be linked to breathing issues.

In general, variations should be low - so seeing frequent, large variations could be a sign that you may be experiencing potentially serious breathing disturbances during sleep.

Fitbit does note that its customers should always consult your health care professional about any questions or health issues you may have.

You can find out more about the Fitbit range in our Fitbit buying guide, which includes a detailed feature-comparison table so you can learn which one is best for you - and find the best Fitbit deals to save yourself some money.