Just an FYI mine doesn't make sense even with the way they do their math for the RMSSD calculations for HRV and the fact that i have sinus tachycardia with a constantly high heart rate (but also arrhythmia, a murmur, and dysautonomia so the number should fluctuate too). I'm almost always exactly at 7ms. With my average heart rate, while sleeping, falling within 87 bpm (rare to go that low) and 95 (usually during a migraine) and almost always 90-91 bpm on an average day, 7ms with a random 8ms sprinkled in once a month or so for good measure doesn't make a lick of sense. I have EKGs during surgery and using the RMSSD method of calculation that still doesn't make sense. It should be a lower number than an average person by far but not THAT low.
Take it with a grain of salt. What I would say is to really just pay more attention to it if it's suddenly weird/out of range for you based on whatever Fitbit has decided your numbers/range should be. In a way it is like the spO2 - normally you get that measurement at a doctor's office from a finger reader. I run between 97%-100% on average, but Fitbit has my back-of-wrist reading personal range between 94%-98%, and last night I was at 95%. Totally fine. I've been below average too, during periods when I've had asthma attacks.
Keep in mind where the readings are being taken from; you wear it like a watch. When you normally take readings for things regarding your pulse or blood pressure using a wrist cuff it is on the palm side of your wrist. Also, when I first got my Sense, about a month or so later I shaved the hair off my arm where the sensors go and my readings were wildly different for a week as I was born unlucky and hairy like a Wookie (thick black body hair). Did it again this past summer when my wrist area had gotten darkly tanned and the readings changed again. Also tried wearing it on the palm side of my wrist (while being VERY CAREFUL not to damage the watch) and readings were all over the place as I believe the sensors might be too sensitive for data on such thin/translucent skin right against those veins and arteries. The only thing I have not done to test it out for discrepancies is run all these same different style 'tests' on my right wrist as opposed to my left as I have a chest port on that side which screws up all my numbers even when my doctors take measurements. It might be worth it though for you to try and run your Sense through a similar battery of tests and see how your data changes.
