when you say 'wilt' do you mean they droop? or fall off, one of the first things i learnt about growing weed, is that it sleeps.. maybe not like us, but they definately rest in the night, i also noticed they grow more at night, like they collect energy from light, and at night, they burn that energy up, and use it all to make space for more, at least thats my theory.. plants do sleep though, the leaves in the day will be pointed up, and at night, they will rest their little muscles and go droopy, they also produce co2 at night, so they must be doing something very different in the night-time, all my plants go to sleep, and i find they dont like loud music or wind (fans) when they sleep, just like humans..

hope that helps, at least u kno its nothing serious, happy growing!!!