The Forerunners got "breathing variations" page on the sleep widget in the last firmware update, but there's been very little documentation on what it's supposed to do and how to interpret the results.
The new Forerunner 970's user manual has the following to say about breathing variations, which helps at least a little:
"The optical heart rate sensor on the Forerunner device has a pulse oximeter feature that can measure your overnight breathing variations. Breathing variation insights are provided to enhance awareness of your sleep environment and general wellness. Occasional or frequent breathing variations may be due to your individual lifestyle factors or your sleeping environment. Contact your doctor or healthcare provider if you're concerned about your level of breathing variations."
...
"On your Garmin Connect account, you can view additional details about breathing variations, including trends over multiple days."
