They say that dawn light adds a whole new dimension to all forms of photography. Earlier in the day there is less dust in the air than later and so you get a clarity that dissapears as the day progresses.
By midday, the sun is directly overhead so there are no long shadows for the photographer to play with. Nonetheless, you can take photos at this time of day. Indoor shots can be particularly good at noon because this is when there is the most ambient natural light available.