The night or evening time; often used in polite/formal phrases (e.g., apologies for contacting at night).
夜分に失礼しますが、少しお時間よろしいでしょうか。
Sorry to bother you at this late hour, but do you have a moment?
夜分遅くまで勉強して、翌日疲れが残った。
I studied late into the night and felt tired the next day.
夜分帯は交通機関の本数が少なくなることがある。
At night, the number of public transport services can decrease.