We've done this before, usually 8pm-1am. I can sleep well in cars, unless I'm driving.

Liebs stopped car sleeping easily age ~3. It's more distracting having him chatter at night, or playing I-spy. Seriously, we do that journey a few times a year and only once has he slept on it in the last 4 years. It is something to bear in mind, what to do if children don't sleep.