We leave at 4 a.m. and my youngest (7 now) NEVER goes back to sleep once we are on the road. He just can't sleep sitting in the van. He'd thrash all night if we tried to drive through, so that would never work. Fortunately, all three of my kids are really good on vacation drives (can't say the same for at home though). I do enforce the "quiet" rule until 7 a.m. when we stop for breakfast sandwiches and coffee. I'll play one movie before lunch and one movie after. Some afternoon quiet time helps for anyone who needs a nap. They all have their Gameboys, books, and now ds(11) has his IPod. We usually stop every 3 hours for a few minutes (gas, bathrooms, coffee), but as they have gotten older, we can sometimes stretch out the stops. We like to make our stops at Cracker Barrels for clean restrooms and coffee to go for my dh(driver).



And then slap-happy will occasionally kick-in.... thats alwasy fun, too! 
...slept a good deal of the time, too. (as others have stated...)
....I will leave around 6-7am-ish...and have my red-bull or rockstar...vitamin-caffeine-upper drinks, at my side.