small hours
noun [ plural ] uk/ˈsmɔːl ˌaʊəz/ us/ˈsmɑːl ˌaʊrz/ (also wee small hours, early hours); (US also wee hours)
凌晨
the early hours of the morning, between twelve o'clock at night and the time when the sun rises
She was up until the small hours of the morning trying to finish her essay. 为了写完论文，她一直熬夜到凌晨。