We demonstrated successful operation of an NbN single photon detector in the temperature range from 6 K to 1.2 K using a 3 He sorption refrigerator combined with a pulse-tube mechanical cooler. The detector was read out either by microwave amplifiers or by a broadband SQUID-amplifier that limited the maximum counting rate to 10 7 counts per second. This counting rate was only one third of the maximum rate provided by the detector. Besides an increase in the quantum efficiency in the visible and near-infrared spectral range with the decrease of the operation temperature, we found a more than twofold improvement in the energy resolutio n as compared to earlier demonstrated 1 eV at 6.5 K. The noise equivalent power estimated at 4.2 K for visible light was better than 10 -18 W Hz -1/2 . We verified that the lowest achieved dark count rate was still caused by the harsh electrical conditions in the mechanical cooler. Keywords: Superconducting single-photon detector, visible and near-infrared spectral range, energy resolution