I've been feeling tired lately, more tired than usual, so I spoke to one of my regular nurses and she suggested that they do a sleep study; they'll attach a pulse oximeter to measure my blood oxygen saturation as I sleep. It then occurred to me that I wear a Garmin fitness tracker in bed to monitor my sleep and it measures my pulse and oxygen saturation, too.
After reviewing the data from the last three nights, I saw that my average heart rate is 59 beats per minute, whereas the lowest was 47bpm which seems pretty low to me. The oxygen saturation was an average of 90%, which isn't great at all, and the lowest was 82% which, quite frankly, is a bit shit.
The Garmin website states: “the data and information provided by these devices is intended to be a close estimation […] but may not be precisely accurate” so, while they're not exact, they can't be that far out, but I'll wait until the sleep study is done (and I've had my upcoming respiratory physiology appointment) before I start to worry too much.