Earlier quoted context omitted.
Dunno about crawly things, but mosquitos definitely first go for body parts not covered by sheets.
which is pretty ineffective since mosquitos are quite good at finding the body part that's uncovered. If you sleep like a cocoon and don't move i guess that works.
Obviously for use in the hot summer; also if you sleep with a fan pointed at you in Korea, you'll catch fan death and die. Cultural myths about fan death is an interesting sleep related topic. With respect to nature vs nurture, different cultures certainly do weird things such as fan death related to sleep but the vast majority cross culturally like blankets; that would imply blankets could be genetic in some weird way not merely cultural.