Well, I could do it with a Kinect if you'd prefer. Or probably with a phone accelerometer - if you actually keep that in your pocket while you're sitting on the couch for an hour! But at some point the data has to come from somewhere.
If you're using a Fitbit anyway, an on-body sensor is going to be more accurate than not, and it's there.
I understand the problem, I'm just saying, when you have a stream of data like this coming in then it's not a difficult thing to do. Nor would I see it ever having any practical application since, well, I doubt people will be hooking up to a fitbit so that netflix can pause their movie when they fall asleep.
If you're using a Fitbit anyway, an on-body sensor is going to be more accurate than not, and it's there.