noun, adverb
from morning till night; day and night; always
Literary or archaic expression for continuous effort or state throughout the day and night. In modern Japanese, 朝から晩まで or 日夜 are more common.
彼は夙夜、研究に励んだ。
He devoted himself to his research day and night.
夙夜の努力が実を結んだ。
His tireless efforts, from morning till night, bore fruit.