perhaps a mouse?

A lot of worms, mites, and other pests like to feast on plants specifically at night. If you don't use an insecticidal soap or other insecticide, i would recommend one.
Another problem could be root rot. If you are overwatering the tomatoes the roots could disintegrate and cause the leaves to shrivel up and fall off. I don't think there is much you can do, if at all, to cure root rot. If that is the problem, try finding a soil that drains better, or just not watering them as much.
One last thing: What is the average temperature where you are? Tomatoes are very sensitive to cold climates. You would think they would be fine in a green house, but you never know.
Wishing you the best of luck,
Janie_appleseed