When is the best time to test for the presence of starch in a leaf

A leaf makes starch when it photosynthesises in light. By midday it has had several hours of sunlight, so it holds enough starch to give a clear blue-black colour with iodine.

Before or just after sunrise, the plant has used up its starch overnight, so the test would be weak or negative. At midnight no photosynthesis is happening.