  Light pollution is the alteration of the natural levels of darkness by an
increased concentration of light particles in the nighttime environment,
resulting from human activity. Light pollution is changing in a deep way the
environmental conditions of the night in wide areas of the planet, and is a
relevant stressor whose effects on life are being unveiled by a compelling body
of research. In this paper we briefly review the basic aspects of artificial
light at night as a pollutant, describing its character, magnitude and extent,
its worldwide distribution, its temporal and spectral change trends, as well as
its dependence on current light production technologies and prevailing social
uses of light. It is shown that the overall effects of light pollution are not
restricted to local disturbances, but give rise to a global, multiscale
disruption of the nighttime environment.
