No, it would not be more sensible to have Earth Hour during the daytime. Earth Hour is all about conserving energy by turning off lights and things that can be done during the night time. It would not make much sense to do this during the day. Earth Hour would be better at night.
No, it does not make sense to have Earth Hour during the daytime, because one of the major points of Earth Hour is to practice conservation by turning off lights. It does not make any sense to have Earth Hour during the day, when people do not need to make any sacrifices because they do not need lights on in the first place.