Evolution with time of the charge in the muon ring pixels for fixed radius of r = 1.2° for SST-1M-1 (blue points) and SST-1M-2 (orange points). Each data point corresponds to one single observation night. The green-shaded area indicates the time period of the Crab Nebula observation campaign presented in this work. The linear fits (dashed for SST-1M-1, dotted for SST-1M-2) enable the estimation of the relative decrease in optical efficiency over time.