Cover the bottom of a 9x13 inch baking dish with tomato sauce. Add 1 layer of lasagna noodles, 1 layer of spinach, then 1 layer of mozzarella. Add a layer of tomato sauce, then repeat the three other layers again. Add a final layer of noodles, then cover with the remaining sauce and top with the last of the cheese. Bake at 375 degrees for 35 minutes.