What is breakfast ?
The actual meaning of Breakfast is the first meal taken after rising from a night's sleep, most often eaten in the early morning before undertaking the day's work. However, I have seen some native English speakers claim that "breakfast" can be used to refer to this meal or to refer to a meal composed of traditional breakfast foods (such as eggs, oatmeal and sausage) served at any time of day.
In US, some restaurants offer 24 hours breakfast. I believe, it is the main cause in changing its meaning from early morning into anytime of day.