I know you understand but let me make it more concrete because I think this concept is extremely important. Any device warning the driver that he/she is falling asleep will be nearly useless. Reason is because drivers usually already know they are falling asleep. So do drivers always pull over when they are starting to fall asleep? Sometimes. Sometimes not. Why not? Because when drowsiness sets in cognitive capacity has already been compromised such that proper decision to pull over is made much less frequently. Someone sober may say 100% of the time that they plan to not to get into a car and drive if they get intoxicated in a party. The same person already intoxicated will not be making the same decisions nearly as frequently. So some sort of a beeping sound to alert the driver that they are falling asleep is already too late for the same reasons.
CRAM has the sensitivity to alert the drivers an hour or two before they actually fall asleep while their better judgement capacity is still intact. So a warning goes off plenty of time before the really dangerous period and the driver is able to make preparations to pull over, have a cup of coffee and take a short 20 minute nap. And viola, we have safer trucks on the road.