What you're describing seems a bit more extreme than being simply a matter of preference.

It's not like she couldn't sleep without me before I moved in.

I might be off base here but my hunch is that this is about her subconsciously feeling hypervigilant around sleeping when there is someone else in the house/ someone else with access to her bedroom.

It probably comes from something in her childhood - I'm not saying it's necessarily something like abuse, but somewhere along the line she has gotten the message that the situation isn't safe.

This is likely an emotional issue that she would benefit from dealing with, and you can probably help her with it, if she is able to recognise that staying up to the point of exhaustion isn't ideal for her. If she accepts that, she can take steps to make herself feel safer in bed by herself.