。
She isn't used to staying up late at night.
Literal
[topic-は] night late until is-awake [in-のに] is-not-accustomed.
慣れる ('to become accustomed') takes に for what one is getting used to — usually a state, activity, or person. Here the activity is wrapped in の (the nominalizer) and marked with に: 起きているのに慣れる ('get used to staying awake'). This のに is the nominalizer + locative に, NOT the conjunctive ~のに ('despite'). 起きている describes the ongoing state of being awake, not just the moment of waking — 起きる as a change-of-state verb means 'wake up,' so its ~ている form means 'is awake' (resulting state).