Left panel, top: average CARMENES stellar spectrum on the first night around the Na I D₁ and D₂ doublet. Left panel, bottom: average of all sky spectra (uppermost spectrum in black), and the lowest and highest airmass points, respectively (second (blue) and third (red) spectra, respectively). To avoid any offset between the wavelength solutions in fibers A and B, we use the pixel grid instead of wavelength for the x-axis. The profiles are shifted by an arbitrary offset for clarity. Right panels: same as left panels but for the second night.