We analyzed spectra of light and heavy water at temperatures from 4 up to 50 degrees C in a frequency range of 0.15 to 6.5 THz. It was shown that the amplitude of high-frequency relaxation absorption band with its maximum at 0.5 THz extends with increasing, temperature and this temperature dependence for light water has a marked feature at 35-40 degrees C as a sharp growth. This fact is noteworthy because this range corresponds to physiological values of a body temperature of the warm-blooded organisms. At the same time, the analogous temperature dependence for heavy water in the considered temperature range lacks this particular feature. Thus, the water with its properties differs significantly not only from other fluids, but also from its own isotopologues.