Not all of them. Snowy Owls and Burrowing Owls are diurnal, and there are some more, but those are the 2 I'm most familiar with.

Barred Owls are very adaptable and tend to do whatever they want. Though able to be strictly nocturnal, they can also be crepuscular, most active at dusk and dawn. A lot just depends on the prey activity where they are. When they have babies, they will hunt into the daylight if required.

Animals also don't get nice blocks of uninterrupted sleep like we do, as they still have a lot of dangers to be aware of. I found one study of a young Barred Owl where it would be awake almost 30% of each hour during the day.