Remove 2 teaspoons zest and squeeze 2 tablespoons juice from lemon. For falafel, in a food processor combine the zest and juice, and the next eight ingredients (through cumin). Cover and process until finely chopped and mixture just holds together (there should be some visible pieces of garbanzo beans).
Using your hands, shape garbanzo bean mixture into four 1/2-inch-thick oval patties. Coat a large nonstick skillet with cooking spray; heat skillet over medium-high heat. Add patties and cook 4 to 6 minutes or until browned and heated through, turning once.
Open pita halves to make pockets. Fill pockets with spinach, tomato slices, and cucumber slices. Add falafel and top with yogurt sauce.