Mix ricotta cheese and eggs together in a medium bowl until blended. Stir in spinach, mozzarella cheese, 1/4 cup Parmesan cheese, sugar, salt, and pepper. Stuff mixture into uncooked pasta shells.

Pour 1/2 cup spaghetti sauce into a medium baking dish; spread to cover the bottom. Arrange stuffed shells in a single layer over sauce. Pour remaining sauce over shells. Cover and chill in the refrigerator, 8 hours to overnight.

When ready to bake, preheat the oven to 400 degrees F (200 degrees C). Remove shells from the refrigerator.

Bake, covered, in the preheated oven for 40 minutes. Uncover, sprinkle with remaining Parmesan cheese, and bake for 15 more minutes.