Sirius and lily were often found in the common room at ungodly hours for one of these reasons

•Nightmares (Sirius)
•can't sleep (lily)
•Probably devising a scheme to get James and Remus together [both]
•etc

But the most commonly known reason is namely because Sirius just wants someone to read with him because being alone in the tower was terrifying even though you knew people were sleeping inside, the silence was deafening and it made him feel like he was in Azkaban.

Of course, sleep talking with lily is also a factor