This phrase is literal in origin and in use. That moment when day breaks the night sky. It's actually one of my favorite moments of a 24 hour period, I really treasure the peace of daybreak/dawn. But a lot of times we use the phrase to complain about having to get up early to get to work.

To crack someone up is an American idiom which comes from the 1600’s.  At the time, make up was applied so thickly to a woman’s face that if she laughed it would crack up.  When the phrase it used today it is to indicate laughter - hard laughter that can result in stomach pains, sore cheeks, teary eyes.