I have a 6:00 AM flight from Marseille to Paris in March, and then an onward connecting flight to the US. I'll be staying at a place in Marseille that is 40 km away from the airport. Would it be advisable to try to go to the airport the night before around midnight, and stay in the airport? Would that be allowed?
Sleeping In Airports suggests that you can sleep on Floor 2, Hall A, Terminal 1. Everywhere else in the airport is closed overnight.
Only you can judge whether that is advisable. Other options include a taxi from your existing accommodation, or taking a room at a hotel nearer the airport. There are several listed on the same page, all claiming to be 3 minutes from the airport. One offers a 24-hour shuttle service (the others only operate from 5am to midnight).