In Sisyphus cooling, the two hyperfine ground states of  experience equal and opposite AC Stark shifts from the near-resonant counter-propagating beams. The beams also effect a polarization gradient, alternating between linear and circular polarizations. The potential energy maxima of one  coincide with pure circular polarization, which optically pumps atoms to the other  , which experiences its minima in the same location. Over time, the atoms expend their kinetic energy traversing the potential energy landscape and transferring the potential energy difference between the crests and troughs of the AC-Stark-shifted ground state levels to emitted photons.
In contrast, gray molasses only has one sinusoidally light-shifted ground state; optical pumping at the peaks of this potential energy landscape takes atoms to the dark state, which can selectively evolve to the bright state and re-enter the cycle with sufficient momentum. Sisyphus cooling is difficult to implement when the excited state manifold is poorly-resolved (i.e. whose hyperfine spacing is comparable to or less than the constituent linewidths); in these atomic species, the Raman-type gray molasses is preferable.