Day and night; the period of daytime and nighttime, or continuously without stopping (e.g. day and night work).
彼は試験前に昼夜勉強した。
He studied day and night before the exam.
工事は昼夜を問わず行われている。
Construction work is being carried out day and night.
昼夜の差が激しい地域では服装に注意が必要だ。
In regions with big differences between day and night, you need to pay attention to clothing.