Introduction Lyman-$\alpha$ spectra can provide insights into the small-scale structure and kinematics of neutral hydrogen (HI) within galaxies as well as the ionization state of the intergalactic medium (IGM). The former defines the intrinsic spectrum of a galaxy, which is modified by the latter. These two effects are degenerate: While at low redshifts, double peaked Lyman-$\alpha$ spectra are the norm, we find more and more spectra only showing a red peak at high redshifts.