watermeadow, I'm sure your friend isn't anything like the late Princess Margaret who made everyone in her company sit up with her into the next day, that would be just too imperious
I'm sure your friend will understand, as indeed most of us do, that not everyone is on the same time frame and some people just cannot stay awake into the small hours we don't all have the same physical constitution for various reasons.
We've sometimes gone to bed before guests, they keep coming back so clearly not a problem, it isn't rude imo you just pose it diplomatically and I'm sure she'll understand. From your original post it's seems she's the one that's asked if she could stay with you not the other way round, so really I wouldn't worry stick to your routine.