REM sleep and FitBit

[selfcoacher]

Some general pointers
REM - variable and higher heart rate and variable and higher breathing/respiratory rate. Used median lines and zoom y-axis for better view of the slight differences. Lower amplitude flow rate. More and higher number of flow limits.
Deep - steady and lower heart rate and steady and lower respiratory rate. Less or no flow limitations. No movements. Clear and steady flow rate.

You can see fitbit got wrong the timing of your first 2 REM stages.

@Geer1 what UF1 and UF2 settings do you use?

[PurchanceToDream]

Fitbit may be pretty OK tracking sleep stages for people with healthy normal sleep--and that is almost certainly the type of person for which studies were done on its accuracy.

I think if you have any form of disrupted sleep, including apnea, it's going to have a lot more trouble accurately tracking sleep stages -- especially distinguishing REM from Awake. That is my experience currently. I have it showing periods of REM where I know I was awake(i.e. I did something to specifically note the time when I was awake), and vice versa(harder to verify). If your treatment is working well and you are sleeping soundly through the night, then it may be more accurate.