See I thought it would measure input and response but I think it does a calculation. I think it calculates your responsiveness plus drivetime.
I took a long road trip with it on and after three hours I was still very awake and attentive but it pinged me to take a break. So it seemed it was really going off distance/time. It probably takes movement into consideration but I'd say it has programming that tells it "hey, they should be taking a break now".
Also mine was on from the factory.