To prevent pillows from yellowing, they should be regularly cleaned with bleach. The yellowing of pillows is mainly due to sweat secretion or perspiration during sleep. These fluids create an environment conducive to the growth of bacteria and mites. These pathogens thrive in moist environments and can proliferate in your bedding. While some people might throw away their pillows at the first sign of yellow stains, and others believe pillowcases are sufficient to protect their health and extend the life of their pillows, it’s actually recommended to wash them at least every six months. The duvet can be washed once a year. Yes, your pillows can be machine washed.