Median stacked spectra obtained for the high-mass sample in each of the four redshift bins, normalised to their mean value in the blue band of the D_n4000. In grey, we show the two windows in which the red and blue bands of the D_n4000 are defined, while horizontal lines represent their median values in these two ranges. We remind the reader that the median spectra have been normalised in the blue window; therefore, the average fluxes of the four spectra in that region are identical by definition.