I confess that long journeys on motorways are hopelessly soporific for me.
It's a nightmare.
More than once I have suddenly realised that the lorry on front is suddenly nearer than it was, and that because I have been somewhat too absent.
I worked with a chap who lost his whole family in a car crash because he fell asleep at the wheel, so these things are to be taken very seriously.
What to do ?
Stop at the first services and drink a nice strong cup of coffee. Then take a short nap. In the car is OK, or in a quiet corner of the café. Then use the loo and resume your journey.
This advice, which I heard some years ago, may have saved my life. Who knows.
Read an article about it here.