keep late hours
To go to bed late; habitually stay up (and work) late.
"If you always keep such late hours, your health might suffer," Tom's doctor said.
on the hour
Each time the hour has zero minutes and zero seconds.
The uptown bus goes past the school on the hour.
The woman must take her pill on the hour.
small hours
The very early hours of the morning between 1 and 4 A.M.
My brother was in trouble for coming home in the small hours.
strike the hour
To mark or toll the hour (said of clocks or bells).
We heard the church clock strike the hour of two.
until all hours
Until very late at night.
He is so anxious to pass his exams with flying colors that he stays up studying until all hours.
wee hours
The crack of dawn, or just before it, usually between 1 A.M. and 4 A.M. or 2 A.M. and 5 A.M.
He stayed up all night when they were expecting their first child; finally, a boy was born in the wee hours of the morning.
zero hour
The exact time when an attack or other military action is supposed to start.
Zero hour for the bombers to take off was midnight.
The time when an important decision or change is supposed to come; the time for a dangerous action.
It was zero hour and the doctor began the operation on the man.
On the day of the championship game, as the zero hour came near, the players grew nervous.