to be industrious and work hard from midnight until dawn; a variant form of the idiom describing diligent study

Burning the midnight oil and waking with the rooster; describes someone who studies or works with extreme diligence and hardship.

(literary) People of great age and experience whom the emperor would formally honor as 'father' and 'elder brother' to demonstrate filial respect and set a moral example for the country.

The period of time just before daybreak, typically referring to the hours between 3 a.m. and 5 a.m.