This morning when I was waiting for the bus I noticed that the sky was starting to get lighter. Twenty minutes later there was definitely enough light to see my way around and do anything except read the newspaper. Another ten minutes and I could read. A further 25 minutes later the sun actually came up.
So when was daybreak, and why? Do you use the word? What do you mean by it?
ETA This morning it was overcast, no colour in the sky at all, and the amount of light three quarters of an hour after sunrise was about the same as half an hour before sunrise yesterday. The sky scarcely got pale at all for ages, then I was vaguely aware the east part was a bit lighter than the west. So today it got light enough to see things clearly about 40 - 45 minutes later than it did yesterday, though sunrise was the same time!
Decision: I think I shall continue to avoid using the term. Far too ambiguous.