c.1300 (as two words from mid-12c., daies liht), from day + light (n.); its figurative sense of "clearly visible open space between two things" (1820) has been used in references to boats in a race, U.S. football running backs avoiding opposing tackles, a rider and a saddle, and the rim of a glass and the surface of the liquor. The (living) daylights that you beat out of someone were originally slang for "the eyes" (1752), extended figuratively to the vital senses.
A clear and open space between two things, horses, players, boats, etc: Daylight began to open between the two leaders/ He went into the line, but couldn't find any daylight (1820+)
To work at a second job during the day: who is daylighting in an ad agency as a producer of commercials (1970s+)
[verb sense based on moonlight]