When I put my head on a pillow in my bed of slumber, moisture exudes from the back of my neck in the form of sweat or more delicately, perspiration. This liquid is then soaked up by the pillow. In the cold or temperate countries in which I have suffered during the long journey of misery and pain, this process does not appear to influence things much as I guess the sweat is just absorbed and gently stored in the pillow. In Okinawa it is different. Indeed sweat is gently absorbed by the pillow but the day of reckoning comes during the rainy season during which both temperatures and humidity are very high. This combination turns anything vaguely organic into mold . The downy pillow on which you lay your weary head, once the lime scented pillow case has been removed, turns out to be a hotbed of bacteria, mold , slime, ectoplasm etc.
I recently had guests and when preparing their sleeping stuff, found that all my pillows were inhospitable. I went therefore to the pillow shop and bought a lot of Japanese hardcore pillows. These are filled with grains of buckwheat. I think the idea is that the wind howls through the grains better thus drying out the sweat. Also there is an indicator of worn-outness when the grains begin to sprout in the wet and humid condition inside the pillow.