> For the complete documentation index, see [llms.txt](https://docs.tryterra.co/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.tryterra.co/faq/help-topics/data-api-sdk/provider-coverage-and-per-provider-data-availability/cgm-spo2-hrv-sample-regularity.md).

# How regular are SpO2 and HRV samples from watches?

Sample regularity is determined by the device. Apple Watch records AFib classifications only during ECG readings, SpO2 more frequently, and HRV at arbitrary rest intervals per Apple's own logic.

These intervals **cannot be made consistent** from outside, and Terra passes through whatever the watch records.

An unsupported CGM brand cannot be connected until Terra adds the provider.


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