[Mr.GoodStuff]

Curling leaves are a plant response to high temperatures. The plant cannot absorb enough water so it curls the leaves to get less light (and heat) to the sensative leaf tips.

I imagine you either are experiencing high temperatures (something over 80) or you aren't using fans to cool the leaves. I've had temperatures in the 75-78 range cause problems without the use of fans to cool the leaves.