I spent two weeks that exact summer (2002) with my then-fiancee (now my wife) where we noted on an early cab ride to Barcelona airport at 5:00 AM. No one was home yet, it was like noon, everyone was out in the street, partying.
You are correct. Funny, same observation.
Loved Barcelona and Seville.
That much partying would kill someone who had to work for a living.