I'm surprised no one's mentioned this yet. Roll down the windows and get some fresh air in the car. Keep the A/C off recirculation mode. Stop trying to breathe CO2.
This will be the first year in the last fifteen that I haven't driven at least 50k miles for work. I finally pulled over and took a nap for the first time ever a few weeks ago. I sleep horribly in hotels but this one was bad. So no sleep for two days plus mind numbing interviews and I was beat. Left Chattanooga just after lunch and by Huntsville I was in bad shape. Pulled over and set the cell phones for thirty minutes. Felt like a new man when I woke up. No issues for the rest of the ride.
Back to the open windows...my opinion is modern cars are simply sealed too well. Not enough exchange of air. Soon enough your just breathing what you exhale. I feel myself nodding off I pull down a window. Couple of minutes later and I'm good to go.
One more reason to love my Porsche. All that fresh air leaking in.
I drove LA to Houston in three days. Day two was 16hrs of driving. My co-pilot was 9yo so I did it all. Never once got drowsy. Water, snacks and coffee. Stay away from heavy meals. Get out, walk around a bit. Blast some tunes.
J