  We discuss particle production in spacetimes endowed with a universal horizon
in Einstein--Aether and Horava gravity. We argue that continuity and
differentiability of the lapse function require the orientation of the
foliation in the interior of the horizon to be reversed with respect to the
exterior one. Unless this is allowed, interaction of gravitating scalar fields
with the universal horizon leads to unitarity violations in the quantum theory.
This property is responsible for particle production by the universal horizon,
as we show by computing explicitly its Hawking temperature for all stationary
and spherically symmetric spacetimes. We particularize our result to known
analytic solutions, including those compatible with observational constraints.
