Night life starts in "Sin Street" between 10 and 12 pm. In Summer, there is action almost every night. Maybe Mondays are somewhat quieter. Bars and pubs in "Carrer del Pecat" are full till 3-3h30 am.
If you want to experience the real Sitges party, you will have to head to one of the discos in town. The two most popular clubs in Sitges are Pacha (which I mentioned before) and Atlantida. This later disco is quite far from the city center. You have to take a cab (or your own car) to get there. The action starts around 4 am and lasts till dawn. Techno and house is the music they play. Remember that you have to dress properly to enter. This is one of the most fashionable clubs in Spain, after all.