George Vranek wrote:
Why are record trials of gliders and motor gliders limited by the time from
sunrise to sunset? Motor planes does not have such restriction.

This is not correct. From FAI Sport Code Section 3:

====
4.5.4 Night Flight

A flight which continues beyond the hours of legal daylight in the
country concerned shall not be validated, except where the glider
and pilot comply with the laws of that country for night flight.
====

Night flights by glider are permitted in the US (and likely many
other countries) if the gliders airworthiness certification is
not restricted to day only, and it is equipped with navigation
lights.

Marc