Are there actually rules about which foods belong to which meal?

The idea that eggs and cereal are morning food and pasta is evening food is a cultural convention with shallow roots. Japanese breakfast commonly includes rice, miso soup, grilled fish, and pickles. Turkish breakfast is a spread of cheeses, olives, tomatoes, and eggs. Congee, a savory rice porridge, starts the day across much of East and Southeast Asia. Full English, continental pastry, and American cereal are three different local answers to the same question, and the American version in particular was shaped heavily by early 20th-century cereal marketing rather than by any nutritional logic.

What nutrition science actually says is about composition rather than timing: protein and fiber at any meal blunt appetite and stabilize energy better than refined carbohydrate and sugar, which happens to be an argument against much of what is sold as breakfast food. A bowl of frosted cereal is not more appropriate at 8am than leftover stir-fry, it is just more familiar. The one legitimate consideration is personal, since heavy or very spicy food close to bedtime disrupts sleep for many people, and caffeine timing genuinely matters. Beyond that, breakfast for dinner is not a transgression, and treating meal categories as rules mostly limits what people are willing to cook.