Hey,
It's nothing special, you don't pick up, she gets worried, it's a good sign.
It means she's getting more invested in you.
If you want to sleep and not pick up after midnight, do so...
You also shouldn't apologise for it, since that is you.
But, always ignoring it, might resolve in, she decreasing contact with you, instead of her txting/calling earlier, beware of that.
What a good solution would be, stop hinting about it, and start talking
. Good communication is key in every good relationship.
Cheers!