Porteños know how to party. In a pretty classy way. They know how to dance (guys and girls alike) and how to have a good time.
Clubs in Buenos Aires open no earlier than midnight and lines don't really form until around 2:00 am. Then it's a time warp with bands and lights and dj's and dancing. Closing time isn't until sometime between 7:00 and 9:00 am and it is perfectly normal for the average Argentine to stay out that late (early, rather) and catch a train home with commuters off to work. And then sleep the next day away... Fortunately for many students, classes at the public university often start in the late afternoon; this schedule caters well to a thriving nightlife.
I will never forget my first morning in Argentina, when I struggled to get out of bed before noon. I finally awoke and took a hurried shower, worried about being rude and making a terrible first impression. When I found my host mother, I apologized for sleeping so late. The confused expression on her face was priceless. She informed me that noon was not late at all. In fact, she had just woken up and the rest of the family was still sound asleep. I figured this was something I could get used to...