Best response to “are you up?”

On the off chance she wanted to hook up just say "i was but i dont check this app often. Shoot me a text next time" and add your number.

Most likely she was bored and wanted to talk. You can either just say yes or call her out.

"I got quite busy last night and didn't check this. Shoot me your number so we don't miss out on a late night Monopoly game"

"was sleeping but you were in my nightmares"

"yes, what are you doing up past your bed time anyway "