That was my first thought. It's not a good idea to water your plants at night, because without the sunlight and heat of the day the water doesn't evaporate as

That was my first thought. It's not a good idea to water your plants at night, because without the sunlight and heat of the day the water doesn't evaporate as quickly. Water sitting on the leaves all night can cause fungus and such things to grow, which is bad for the plant. Since plants can't avoid being rained on at night, it seems logical that they would evolve a mechanism for staying as dry as possible.