My vegetable plants are wilting. Why?

I have cubers, tomatoes, bell peppers, and peppers. In the morning when I water the plants are standing and perky, but after noon they start wilting by 6 at night they are really pathetic looking. Are they getting too hot and do I need to do anything to protect them. I have them in a 6' X 4' raised bed. I was thinking it was the heat of the day affecting the moisture on the leaves from morning watering, so should I water at night?