Hello Lisa, thanks for stopping by :-)
The reception desk is open 24 hours, and there is always somebody there, even at 4:30 AM when you get home from partying, hehe. But don't party too hard or you will miss breakfast!!!! Anyways, even though the reception is open, you get a magnetic key that opens the door to get in to your bed at any time of the day or night. There are some restricitons as far as staying at the hostel; you can't make a lot of noise after 1:00 AM or use the showers at night, because there are people that are sleeping, which is understandable :-) Also the rroms are cleaned VERY thouroughly in the morning, around 11... ... ... but you're not coming to Barcelona to sleep are you? I didn't think so !
The metro in Barcelona closes at midnight on weekdays, at 1:00 AM on Friday and goes ALL NIGHT on Saturdays. Ot opens at 5:00 AM, I think, so it is fairly easy to reach the Hostel at night or early morning. If the metro is closed when you want to come home, then there is a night bus that starts at Plaza Catalunya (right next to Las Ramblas) and leaves you very close to the Alberguinn, not even 5 minutes walk from where the bus leaves you to the hostel.
Of course, there are always Taxis, which are farily cheap in Barcelona (especially compared to Germany) but the night bus or metro are the good options if you're on a budget.
I hope I answered your questions, if there's anything else that you would like to know, just ask!!!!