There are cards that hang on your door handle that you can fill out each night to request breakfast in the morning. It is not a full breakfast, but you can get cereal, toast, muffins, coffee, tea, juice, etc. We would fill out the card and use the delivery as a wake up call. They usually call your stateroom a few minutes before to let you know they are coming. Don't forget to tip them.
Most of the restaurants are open for set times during the day for both the sit down breakfast and lunch and the lunch buffet. Plutos which serves chicken fingers, burgers, etc. It is open from about 11AM until 10PM. Then there's always room service which is open 24 hours a day.