How to go to the airport in Sofia after midnight?

I would rent a room nearby, in walking distance the night before the flight. Even if not in walking distance, being nearer can make an huge difference in time and ride price. Taxi rides from the center of town in many European cities can be costlier than a flight.

I would not advise spending the night in the airport - when I was younger I made the mistake of sleeping in Heathrow airport with lots of people nearby, and someone, probably an homeless person, stole the ticket from my pocket while I was sleeping, probably for being able to sleep there and move freely without the guards pestering him. Luckily, I had enough money and time to buy a replacement, but it cost me as much as renting a nearby room. Airports waiting rooms are not an entirely safe place for someone alone at night.