Between us, when I read "as soon as your head rocking," I worry.
I think when we are at the point where the head rocking, it far exceeded drowsiness. At this stage, it is already in the set, or built under a truck. Indeed, vigilance is relaxed and it is manifested by more tenuous signs. I'm no expert in this area, but I know that there are systems based on the electrical conductivity of the skin, on the analysis of eye movements, etc. So head that rocks, for me it is a much more advanced stage. If vigilance is relaxed and already it's dangerous, you imagine the time that can elapse between this relaxation of vigilance and when the rocker head?
I think this stuff is to be used for hands-free, but certainly not to get out of danger facing the problem of falling asleep at the wheel!