The four-letter word that describes the light that results from the rising sun is "dawn," according to Oxford Dictionaries. The website indicates that this term refers specifically to the first appearance of the sun's light before the sun is visible.
The common expression "from dawn to dusk" refers to an activity or endeavor that requires a commitment to work from the first light of the sun to the last. Oxford Dictionaries also indicates that "dawn" is used as a metaphor to describe the onset of a new lifestyle, career, event, pursuit or activity. This application is evidenced in the expression "dawn of a new day."