14 CFR § 107.29 - Daylight operation.
(b) No person may operate a small unmanned aircraft system during periods of civil twilight unless the small unmanned aircraft has lighted anti-collision lighting visible for at least 3 statute miles. The remote pilot in command may reduce the intensity of the anti-collision lighting if he or she determines that, because of operating conditions, it would be in the interest of safety to do so.
(c) For purposes of paragraph (b) of this section, civil twilight refers to the following:
(1) Except for Alaska, a period of time that begins 30 minutes before official sunrise and ends at official sunrise;
(2) Except for Alaska, a period of time that begins at official sunset and ends 30 minutes after official sunset; and
(3) In Alaska, the period of civil twilight as defined in the Air Almanac.