the crack of dawn

The moment at which the sun first rises. It's a long drive, so we'll have to leave at the crack of dawn if we want to get there on time. I've been up since the crack of dawn repairing the fences that blew down in the storm.

Very early morning, daybreak. For example, I got up at the crack of dawn. The crack in this term alludes either to the suddenness of sunrise or to the small wedge of light appearing as the sun rises over the horizon. Originally the term was usually put as crack of day. [Late 1800s]

Early in the morning. The origin of this expression is uncertain. One writer suggests that “crack” is derived from the ancient meaning of a sudden loud noise (since the word comes from Old English cracian, “to resound”), because the sun comes up suddenly. Rudyard Kipling used similar imagery in his poem “Mandalay,” where “the dawn comes up like thunder outer China ’crost the Bay.” On the other hand, “crack” may refer to a small space or opening—that is, the wedge of light that appears as the sun rises over the horizon. Whichever, the phrase originated in America in the late nineteenth century. It may already have been a cliché when W. Somerset Maugham wrote (Catalina, 1948), “He had slipped away at the crack of dawn.”