Fitbit is enabling a blood oxygen (SpO2) monitoring feature on its Versa and Ionic smartwatches and Charge 3 wearables.

News of the new capability, which can help detect problems like sleep apnea, came first via a blog called TizenHelp, but it has since been confirmed by the company.

“Blood oxygen saturation normally fluctuates, but big variations can be linked to breathing issues,” a note in the Fitbit mobile app explains. “Estimated blood oxygen variation approximates the changes in your blood oxygen saturation.” The explanation is provided next to a graph of any given night’s estimated oxygen variation, with a divider between big and small variations.

Fitbit says that comparable wearables use “a combination of the red and infrared sensors on the back of your device, which are part of the SpO2 sensor,” to measure oxygen variation. And while this isn’t necessarily hugely accurate, those how do see large variations almost certainly suffer from some form of sleep disorder.

There’s no way to force the update that adds SpO2 monitoring as far as I know, but it appears to be available now in the United States and is rolling out to those with Fitbit Versa, Versa Lite, Versa 2, Ionic, or Charge 3 devices. Some competing wearables from Honor, Huawei, Huami already offer this functionality, but Apple Watch does not.