1
general::
Phrase(s): dawn (up)on someone
Fig. [for a fact] to become apparent to someone; [for something] to be suddenly realized by someone. (Upon is formal and less commonly used than on.) • Then it dawned upon me that I was actually going to have the job. • On the way home, it dawned on me that I had never returned your call, so when I got home I called immediately.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Also, dawn upon. Become evident or understood, as in It finally dawned on him that he was expected to call them, or Around noon it dawned upon me that I had never eaten breakfast. This expression transfers the beginning of daylight to the beginning of a thought process. Harriet Beecher Stowe had it in Uncle Tom's Cabin (1852): "The idea that they had either feelings or rights had never dawned upon her." [Mid-1800s]
American Heritage Idioms