Why are my tomatos wilting at the same time every day.?

I am growing tomatoes hydroponcily. They are growing in an east window, and have a grow light on them. They are doing very well, but every day around noon when the sun stops shining on them, they wilt. They recover later in the day. This has scared me several times. Why dose it happen, and can something like this affect them in a more significant way later on?