Are sharks nocturnal

Yes, they have retina in eye like other nocturnal animals like the cat which helps it see better in darkness.

Great whites most often feed in the morning, after dawn, and at sunset, when visibility is poor for the prey.

Although great whites can theoretically prey 24 hours a day, nocturnal feeding doesn't seems to be the rule.

However, nocturnal behaviour of Carcharodon carcharias is poorly understood.