Green’s Dictionary of Slang

(US) used of one who works outside their normal hours of practice, e.g. lawyers, doctors (presumably to boost their income).

1897		Boston Transcript 5 Aug. 5/1: There are sundown doctors, sundown lawyers and sundown ministers [DA].
1913		‘Lord Ballyrot in Slangland’ in Tacoma Times (WA) 10 July 4/4: Put on your soup-and-fish, your sun-down rags, your O-Gussie harness.
1940		E.F. Frazier Negro Youth 16: [S]ome families live beyond their means, or, as one investigator found, the head of the family engages in so called ‘sundown’ occupations.
1944		N&Q IV Aug. 71/2: Sundown lawyers: Government employees who practice law after their regular working hours [DA].