Abstract: It is an object of the invention to provide a garment and sleepwear that can improve the quality of sleep, and to provide a method for improving the quality of sleep by using such a garment or sleepwear, and this is achieved by a garment for improving the quality of sleep, the garment including a fiber containing at least two kinds of nanoparticles that emit far-infrared radiation and cause electron transfer based on plasmon resonance.