Simple examples:
* The owl is a nocturnal creature, meaning it is active at night.
* The nocturnal habits of the raccoon make it difficult to observe during the day.
More complex examples:
* The nocturnal atmosphere of the city, with its flickering streetlights and hushed whispers, was both unsettling and alluring.
* While most animals are nocturnal for safety reasons, some species, like the sloth, are simply better adapted to cooler temperatures.
* The nocturnal activities of the thief went unnoticed until the morning, when the damage was discovered.
Figurative language:
* The nocturnal silence of the forest was broken only by the occasional hoot of an owl.
* Her nocturnal thoughts, dark and swirling, kept her awake long after the sun had set.
Remember, "nocturnal" describes something that is active at night, but it can also be used figuratively to convey a sense of mystery, darkness, or secrecy.