It's been getting hot here and I have had the air conditioner on most days. My acoustic lives on a stand in the lounge and I noticed the top had sunken quite dramatically last night. It looked like I had Karate chopped it right where my hand rests on the bridge. It also made a rattling sound similar to fret buzz when played aggressively and I had been playing it more aggressively than usual due to learning Bron y aur stomp and playing the hell out of it to work it up to speed. Anyway I have a bunch of those cheap plastic hydration devices with the foam in them you wet and stick between the strings in the sound hole. Covered with a piece of clear plastic and the top came back into shape and has a nice rise in it. Took about 3 hours to sort itself out. If like me you forget to keep an eye on things you might want to check your acoustic out and see how it's travelling.