day and night; around the clock
A somewhat literary or formal expression meaning continuously, without stopping, both day and night.
The rescue team continued the search around the clock.
Also means 'around the clock', but is more colloquial and common in everyday speech.
From 昼夜 (day and night) + を + 分かたず (negative continuative of 分かつ, 'to divide, distinguish'), literally 'without dividing day and night'.