noun
nightless city; city that never sleeps
Refers to a bustling entertainment district or city where lights and activity continue all night. Often used for historical or literary descriptions of pleasure quarters, but also for modern cities like New York or Tokyo.
かつて吉原は不夜城と呼ばれた。
Yoshiwara was once called a nightless city.
東京はまさに不夜城だ。
Tokyo is truly a city that never sleeps.