  The coupling with the lattice vibrations is shown to drastically modify the
state--of--the--art picture of the excitonic states based on a frozen atom
approximation. The zero--point vibrations renormalize the bare energies and
optical strengths. Excitons acquire a non--radiative lifetime that decreases
with increasing temperature. The optical brightness and efficiency turn out to
be strongly temperature dependent such as to induce bright to dark (and vice
versa) transitions. The finite temperature experimental optical absorption
spectra of bulk Si and hexagonal BN are successfully explained without using
any external parameter.
