  The origin of the universal optical conductivity form sigma(omega) ~
(1-e^(omega/T))/omega established in numerical model studies of doped
antiferromagnets and consistent with experiments on cuprates near optimal
doping is discussed. It is shown that such a behaviour appears quite generally
when the single-particle excitations are overdamped and their relaxation
follows the marginal Fermi liquid concept. Relation to recent ARPES experiments
is also discussed.
