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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.
dawn on you
to suddenly understand something It finally dawned on him that she'd been joking and he was worried for no reason.
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]
dawn onor dawn upon
To begin to be perceived or understood by someone; become apparent to someone: It dawned on me that I had forgotten to pick up some milk. A possible motive for the crime dawned upon the detective.