。
She used to stay up late.
Literal
She [topic-は] before often stayed-up-late.
以前よく + past tense is a standard 'used to' frame in Japanese: 以前 ('formerly') + よく ('often') + verb in past form. The combination implies a habitual past behavior, often contrasted implicitly with the present. 夜更かし is a noun-suru-verb compound — 夜 ('night') + 更かす ('let it grow late') — that captures the idea of voluntarily prolonging waking hours. A culturally salient term, given Japan's reputation for sleep-deprived work and study cultures.