expression
day and night; around the clock
Used adverbially to mean that something happens continuously, without regard to day or night. Often used in formal or written contexts.
工事は昼夜を問わず続けられた。
The construction continued around the clock.
この店は昼夜を問わず営業している。
This shop is open day and night.
四六時中 means 'all the time' or 'around the clock', but is more colloquial and emphasizes constant, uninterrupted action.
A set phrase composed of 昼夜 (day and night) + を (object marker) + 問わず (negative continuative of 問う, 'to ask'), literally 'without asking whether it is day or night', i.e., regardless of the time of day.