actually, the answer to the question lies in the entomology of the word
Break Fast
It is the meal where you break your fast (a period of not eating), and as the period where you are sleeping is (for most people) the longest period without a meal, the first meal after a long sleep is considered to be the one to Break your nightly Fast.
Thus, no matter what time of day it occurs at, the first meal of the day is ipso facto Breakfast.