I'm one of the people that jodog hates. I can be perfectly functional for 40+hours straight, and then crash for 48hours and not wake up.
In my case, my youngest child had breathing difficulties when he was born. He was making too much mucus while sleeping and suffocating himself. Anyone reading this will know that as a parent, you'll do whatever the hell it takes to put that back of the right path. So I did. For 3 years & 2 sets of operations. For at least the following 6 years he had the "wonderful" habit of running into the bedroom and diving on top of me, so I developed the habit of sensing him coming and I'd be awake if he was within a 6ft radius of me.
Now, if the sun is up, even with black out curtains, I'm awake. Nothing I can do about it. He's now 18 years old and loves nothing more than sleeping in until 6pm - so I delight in waking him up at 7am.
My answer to the thread - if you have a reason to stay alert and responsive, you'll stay awake. If you don't, an ambulance crew will likely pick you up. Please don't make them pick you up, pull over and check into a motel/hotel before that happens. Recognise the signs of inattentiveness early, and stop riding. There's no shame in it.