  DANSS is a spectrometer of reactor antineutrino based on plastic
scintillator. The sensitive volume of the detector is made of 2500 polystyrene
based scintillator plates with wavelength shifting (WLS) fiber readout
(strips). We present a study of the strips light yield during 6.5 years of
DANSS continuous running. Overall ageing at the rate $0.55 \pm 0.05(syst.)$ %
per year is observed that is considerably smaller than in other similar
experiments. We also observe the WLS fiber attenuation length shortening at the
rate $0.26 \pm 0.04(stat.)$ % per year.
