Why do some (as i only know of some) japanese people sleep with masks on at night?

i've seen it a few times on JP shows and know of a few people too, (not well enough to go up and ask them though) just curious as to why. i think i've seen some poeple walk around with them on as wellWhy do some (as i only know of some) japanese people sleep with masks on at night?
Some who have a very bad case of hay fever will wear masks, even at night to keep away from pollen. SOme it's just so that the family members, especially small children don't catch their germs since many families sleep all together until the kids are around mid elementary school age. (some even longer...some families have started adopting the western way of making kids sleep in a different room.. but that is still not popular whith babies)


Also it is manners to wear a mask and protect others from your germs when you are sick. It also helps to keep you from getting even sicker from other germs.


Masks will also be used at school by those who serve lunch.


(way of school lunches are VERY different from North american)Why do some (as i only know of some) japanese people sleep with masks on at night?
Many Japanese wear masks to prevent others from catching their germs and yes it is common to see them walking around with them on too,for the same reason.