A heat retaining sleeve for raising the cold spot temperature of a fluorescent lamp is comprised of a base end, a distal end, and a sleeve body sized and shaped to fit over the end of a fluorescent lamp or lamps where the lamp's cold spot exists. The sleeve creates a heat retaining air gap between the inner surface of the sleeve body and the lamp end or ends with the cold spot, and has sufficient length to so that, when fitted over the lamp end or ends, the cold spot temperature is elevated to a temperature that increase the lumen output of the fitted fluorescent lamp or lamps.