Honestly, I would say the 1am to 5am is a pretty clear sign. Lots of long conversation doesn't necessarily mean there's romantic interest, but staying up that late is generally somewhat self destructive to a regular sleeping pattern and in my experience (having been on both sides of it) that only happens when there's some kind of interest at play.
As to the nature of that interest, unfortunately the only way to find out is to ask. But I would say, if you've been talking that long and intimately, you lose absolutely no face by just addressing it head on. It doesn't have to be as direct as asking her out, but being upfront with something like "Hey, I really enjoy talking to you these last few weeks/months" etc.
I think in these kinds of situation, when someone is sending a lot of signals but not being direct, the best way to find out how someone else is feeling is to put your own feelings out there. (Within moderation, use your better judgement on that etc). I think the main thing is, to do it in a respectful way since you don't know what her situation is exactly.
Let's say she's single and interested, then the ball is in her court to respond. If she's single but on the fence, well then she knows where you stand.
If she's still in a relationship, it honestly raises more questions, like why she is talking to another guy till 5 in the morning. In my experience, that's a sign that things aren't going well in her own relationship and she's desperate for an outlet or to be seen and perceived by another human being. That can sometimes lead to a relationship, sometimes not. But if that's the case, you have a right to know where you stand and what the situation is.
Apologies if this is a long ash response. I've been in situations like this before... Once as someone in your situation, and once as someone in a relationship. Take it from me, things can get very messy if you're not careful and don't protect yourself sufficiently. She might still be figuring out what she wants, so you don't want to put pressure on her but it's also important for you to let her know where you stand if you're feeling a little strung along by mixed signals?