There are different reasons to sleep at the airport. I had my flight from Madrid at 5pm but I've reached the city at 5:30am (night buses are way cheaper in Spain, yay!). So I've decided to go to the airport by foot (I had a lot of time and I wanted to visit my favourite park there (parque de retiro). But when I got there it's was still 11:30am. As there was not even one bench, I've decided to had a nap on the floor. When I woke up, there were people around me, sleeping as well. Honesty? It was way more comfortable than any other sleep at the airport I had :).