Ben Jeffrey wrote:
US Federal Aviation Regulation definition of night: From FAR official
definitions
Night means the time between the end of evening civil twilight and the
beginning of morning civil twilight, as published in the American Air
Almanac, converted to local time.

Equipment requirements for night flight: FAR 91.205(c)
(c) Visual flight rules (night). For VFR flight at night, the following
instruments and equipment are required:

(1) Instruments and equipment specified in paragraph (b) of this section.

(2) Approved position lights.

(3) An approved aviation red or aviation white anticollision light system on
all U.S.-registered civil aircraft. Anticollision light systems initially
installed after August 11, 1971, on aircraft for which a type certificate
was issued or applied for before August 11, 1971, must at least meet the
anticollision light standards of part 23, 25, 27, or 29 of this chapter, as
applicable, that were in effect on August 10, 1971, except that the color
may be either aviation red or aviation white. In the event of failure of any
light of the anticollision light system, operations with the aircraft may be
continued to a stop where repairs or replacement can be made.

(4) If the aircraft is operated for hire, one electric landing light.

I can find no reference in the FARs that further limits gliders to
operations between sunrise and sunset.

Technically, this is not a requirement for gliders. See the full
heading:

91.205: Powered civil aircraft with standard category U.S.
airworthiness certificates: Instrument and equipment requirements.

Note that this is for *powered* aircraft, so would apply to
motorgliders, but not pure gliders. However, if the glider has a
standard airworthiness certificate, it is probably limited to VFR day
only. If it is experimental, it probably has the same restriction; if
not, there would be specific requirements for night flying equipment
which would probably also include cockpit lighting in addition to
position and possibly anti-collision lights.