Futons are generally used with a futon cover, because these make it easy to keep the futon clean. People generally use futons so they can sleep comfortably, stay warm and support their bodies while asleep. People usually sweat during sleep, so futons absorb sweat and collect moisture. Futons have to be hung out in sunlight to keep them dry, this is called "futon-hoshi". If this is not done, ticks and mold could grow on the futon and it will become dirty and insanitary. Futons need to be looked after properly.