It depends on what you're logging.
When you're logging night
flight time, I think you're supposed to use the FAA definition of "night," which is in 14 CFR 1.1:
Night means the time between the end of evening civil twilight and the beginning of morning civil twilight, as published in the Air Almanac, converted to local time.
When you're logging night
takeoffs and landings, I think you're supposed to use the times specified in 14 CFR 61.57(b):
(b) Night takeoff and landing experience. (1) Except as provided in paragraph (e) of this section, no person may act as pilot in command of an aircraft carrying passengers during the period beginning 1 hour after sunset and ending 1 hour before sunrise, unless within the preceding 90 days that person has made at least three takeoffs and three landings to a full stop during the period beginning 1 hour after sunset and ending 1 hour before sunrise, and—
(i) That person acted as sole manipulator of the flight controls; and
(ii) The required takeoffs and landings were performed in an aircraft of the same category, class, and type (if a type rating is required).
(2) The takeoffs and landings required by paragraph (b)(1) of this section may be accomplished in a flight simulator that is—
(i) Approved by the Administrator for takeoffs and landings, if the visual system is adjusted to represent the period described in paragraph (b)(1) of this section; and
(ii) Used in accordance with an approved course conducted by a training center certificated under part 142 of this chapter.
Note that 14 CFR 61.1, which is the definitions section of Part 61, has no definition of "night."