How does fitbit premium calculate "Activity" in the Daily Readiness Score?

[Hebi]

FitBit support has not been helpful on this, does anyone know what metrics are used to determine the "Activity"? The description says "A long, yellow bar indicates a high level of physical activity and **accumulated fatigue**".

My questions are:

1) How does fitbit premium actually determine fatigue?

2) Is it **assuming** that a high step count = high fatigue, or is it an increased # of minutes in the active zone? My "fatigue" score is all over the place and it doesn't seem to reflect how I actually feel. 

3) How does this correlate with the actual physiological events happening in my body?

Right now the fitbit premium scores don't seem very trustworthy, and the descriptions I've read on the internet doesn't get into the weeds of what I'm trying to understand (which is the physiological basis of how they are determining fatigue). 

[SebWotan · Accepted solution]

Hello @Hebi 

I think nobody knows. Fitbit has not been clear how the length of each bar relates to the various metrics and how they are used to compute the final Readiness Score. 

I would guess fatigue is link to the HRV, which is also one of the three components used to compute the HRV. Or maybe Fatigue is linked to accumulated Active Zone Time over the last X days ? Short of someone reverse engineering the Readiness Score or Fitbit documenting it more, this will stay a mystery.