Under the circumstances, the 'no sleeping' rule does not seem reasonable. I agree with PPs who say that OP should find alternative driving arrangements. I disagree that she's the rude one, even though she goes against the group dynamic.
I agree with this.
And as I have said before, I wonder how it would have actually gone over if the OP had tried to refuse the rides on this basis.
If a group of people really consider the drive there and the drive back to be part of the social outing, and they really don't want anyone to sleep, well personally I find that unreasonable especially after a heavy physical activity like a hike. But if that's what they want, well, okay.
But if the drive there and back is part of the social activity, then wouldn't they be very likely to take offense at anyone choosing not to engage in that part of it at all?
If the OP doesn't even know for sure that she will fall asleep, her declining to ride with the group might mean that they lose her company for the whole time (say, an hour). If she falls asleep for 10 minutes they still get her company for 50 minutes. Even if she falls asleep for 55 minutes, they still get 5 minutes of her company that they wouldn't get if she found alternative transportation.
I am very curious as to what would have happened with this group had the OP just flat out declined. It obviously was important to this driver that he do a favor to the group. Giving hospitality gave him pride and satisfaction. And for someone to just reject it, no matter how politely, I just think it would have caused even more group tension.
And in that case...well, I would question just how good of friends these people really were.
It's such an impossible situation. They want her in the car. But not to fall asleep. She can't help falling asleep. But they want her there and there is no graceful way (I can think of) to not be in that car that wouldn't cause offense. And if it's for the sake of socializing with the others, she loses all of it by respecting their wishes to not lose even a little bit by falling asleep.
I wonder if they would have changed their tune and realized this if she had brought it up directly, or if it really would have been worth it to them for her to find her own ride. Or if there would have just been a big stupid fight.
Bad situation, all the way around.