The first and foremost - an owl can actually see in daylight. This is basically a myth that they can't. However, owls are nocturnal in nature, which means they become active in night.

The inactiveness of owl during day time is majorly due to the shape, size and kind of their eyes.

Owls have comparatively larger eyes with respect to their head. Owls are known for their disproportionally large eyes in comparison to their skull. An apparent consequence of the evolution of an absolutely large eye in a relatively small skull is that the eye of the owl has become tubular in shape. This make the eyes of owl fixed that means they are unable to move their eyes from left to right or top to bottom. So, Instead of moving their eyes, owls rotate their head to visualize their surroundings. Owls are capable of rotating their heads through an angle of approximately 270 degress, easily enabling them to see behind them without relocating the torso.

The fact that they have larger, tubular eyes has also affected its vision.

Large eyes let in more light than small eyes. In addition, their pupils can expand greatly at night, enhancing their ability to see in the dark. Inside of an owl's eyes, there are a large number of structures called rods. Rods are sensitive to light, so a large number of rods help owls see at night. Humans also have rods, but we have a much smaller number than owls have. As a result, owls see 35 to 100 times better than humans can in dim light.

But this quality adversely affect the owls during daylight when they begin to get extra lights due to larger eyes and bigger pupils then actually required to see clearly. Also their pupils don't get as small as ours in bright light, so to block out the extra light, they often close their eyes half-way or more. When owls close their eyes half way, they block out the sky and some of the sunlight, but can still see the ground below perfectly. They may look sleepy or even half asleep when really they are wide awake and alert.