This was one of the reasons I loved having a camper van. Just pull over for a nap whenever needed. When you wake up make a quick coffee or a light meal and carry on.
I used to take advantage of the 2 hours free parking at motorway service stations if I was tired. I trundled home from London several times taking a 1h50 minute nap at the first 4 services.
I find it more difficult in a car, not a good place to nap for me. If anything I feel more worn out for it. I find I have to plan better and arrange an over night stop if the journeys over 6 hours.
Stimulant wise I go for coffee and I drink loads of water too which forces me to stop for the toilet. I like to take small savoury snacks and some sweets as well but not many. I only take a little as if I fill up I get sleepy.
This is most of my coffee kit I carry round.
Missing from the picture is a stovetop espresso maker and some other bits n bobs. The mugs
a clever one with a coffee filter press built in. It's good for loose tea as well. It all lives in a box with a few tracker bars and a gallon of water in the boot.
I used to keep a self heating espresso in the glove box. These
can be handy.