very early in the morning · in the wee hours

You went to bed at 2 o'clock in the morning? No wonder you look sleepy today.

The murder happened between 3 a.m. and 5 a.m.

The custom of eating dumplings on New Year's Eve is related to an ancient Chinese method of measuring time; in ancient China, each day was divided into twelve sections: the beginning of each day was called 'first time', which corresponds to the period from 11pm to 1am.