Do not water your bonsai when the sun is hot and high up in the middle of the day, because the droplets on the leaves will act as lenses that would burn your leaves.

Well, this a myth created long ago when agriculture was first developed sometime in 4,000 BC. This myth is not a myth when you think about conservation of water. You can conserve water by watering before dusk and after dawn, because this is the time when the air is cooler and evaporation is lowest so your ground will be wet for a longer time.

However, this is not ideal for the plant, because the plant demands a lot of water during photosynthesis. Photosynthesis is a chemical process wherein carbon dioxide from the air and water taken by the roots is processed to form sugars which are food for the plant. When water is not enough the plant wilts, turgor pressure drops, photosynthesis stops.