In a small mixing bowl, gently whisk together yogurt, milk, pumpkin purée, maple syrup, and vanilla until well combined.
Place oats, chia seeds, and spices in a container that has a lid or cover.
Stir in the milk mixture and evenly distribute the contents.
Cover the container and place it in the fridge overnight or for at least 4 hours.
Serve the oats chilled, topped with walnuts and vanilla yogurt if desired.
Makes 3 servings, approximately 3/4 cup each.