nocturnal

From Middle French nocturnal, from Latin nocturnālis, itself from nocturnus, an adjective built on nox, meaning "night." Nox traces back to the Proto-Indo-European root *nókʷts, also meaning "night."

The word has stayed close to its root meaning throughout — it has always described something belonging to or happening in the night.

It shares its deep root with English "night" itself, both descending from the same Proto-Indo-European word *nókʷts — as do Ancient Greek νύξ, Sanskrit नक्ति, and Proto-Slavic *noťь.