light dawned, the
Understanding came at last, as in They couldn't figure out where they went wrong, but then the light dawned—they'd turned right instead of left. This expression transfers the beginning of dawn to human perception. [c. 1800]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.