Are cats nocturnal?

Are cats nocturnal? It depends on which cat species you are talking about. I will presume that the question concerns the domestic cat.

The domestic cat is not nocturnal. The domestic goes out and is active in the daytime. However, in general the domestic cat tends to hunt at dawn and dusk (crepuscular) and during the night.

It is probably fair to say that the domestic cat becomes more active at dusk and during the night if allowed outside because of it's hunting activities when it is dark. That is not to say that cats do not hunt in the daytime. They do.

Perhaps above all else, the domestic cat is very adaptable and flexible in its behavior. It can form colonies for example as well as be solitary. The prey it hunts is wide ranging. They are "versatile generalists" in what they eat.

Full-time indoor cats will not behave in the same way.  The well known book, Wild Cats Of The World that has a section on the domestic cat makes no mention of the cat being nocturnal. My personal experiences in keeping cats bear this out. This page is based on personal experience.