wee hours
noun [ plural ] US uk/ˈwiː ˌaʊ.əz/ us/ˈwiː ˌaʊ.ɚz/ (also wee small hours, small hours)
凌晨
the early hours of the morning, between twelve o'clock at night and the time when the sun rises
He was up until the wee hours trying to finish his work. 为了完成工作，他一直熬夜到凌晨。