That's also a solved problem - Mercedes cars have "attention assist" which can figure out how attentive you are by measuring how you drive. If you are literally falling asleep at the wheel, the system will get very annoying and insist that you stop for some rest.
Of course, whatever algorithm they are using won't work if the car is driving itself, you can't judge the reaction time if all the driver is doing is holding the wheel.
One of the approaches used on long-distance buses is "track head position and angle." If you're "looking" into your lap or into the ceiling while driving, that's a fairly reliable signal that something is wrong, never mind how you're holding the steering wheel.
The technology is almost certainly there to very aggressively monitor "eyes on the road" and quickly blare alarms if they aren't. It's probably fair to say that would be a very unpopular auto feature.
Of course, whatever algorithm they are using won't work if the car is driving itself, you can't judge the reaction time if all the driver is doing is holding the wheel.