Depending on the location, McDonald's starts serving breakfast in the morning at 24-hour locations anywhere between 4:00 a.m. and 6:00 a.m. The breakfast hours end between 10:30 a.m. and 11:00 a.m. everywhere, all days of the week. Remember that not all 24-hour locations start serving breakfast at the same time.
It's important to remember that McDonald's restaurants work via franchises. The individual owners of these franchises are the ones who decide what the breakfast hours are depending on the location and whether the customers go there after partying or if they go there because people in that area start their day very early.