  The quite different behaviors exhibited by microscopic and macroscopic
systems with respect to quantum interferences suggest the existence of a
borderline beyond which quantum systems loose their coherences and can be
described classically. Gravitational waves, generated within our galaxy or
during the cosmic expansion, constitute a universal environment susceptible to
lead to such a quantum decoherence mechanism. We assess this idea by studying
the quantum decoherence due to gravitational waves on typical microscopic and
macoscopic systems, namely an atom interferometer (HYPER) and the Earth-Moon
system. We show that quantum interferences remain unaffected in the former case
and that they disappear extremely rapidly in the latter case. We obtain the
relevant parameters which, besides the ratio of the system's mass to Planck
mass, characterize the loss of quantum coherences.
