  We derive the evolution of the energy deposition in the intergalactic medium
(IGM) by dark matter (DM) decays/annihilations for both sterile neutrinos and
light dark matter (LDM) particles. At z > 200 sterile neutrinos transfer a
fraction f_abs~0.5 of their rest mass energy into the IGM; at lower redshifts
this fraction becomes <~ 0.3 depending on the particle mass. The LDM particles
can decay or annihilate. In both cases f_abs~0.4-0.9 at high (> 300) redshift,
dropping to ~0.1 below z=100. These results indicate that the impact of DM
decays/annihilations on the IGM thermal and ionization history is less
important than previously thought. We find that sterile neutrinos (LDM) decays
are able to increase the IGM temperature by z=5 at most up to 4K (100K), about
50-200 times less than predicted by estimates based on the assumption of
complete energy transfer to the gas.
