Early in the morning, at dawn, as in It's a long trip, so we'll have to start out bright and early. The bright here presumably alludes to the brilliance of the dawning sun, which has long been noted by poets. [Early 1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.