Why do buddhists believe a dead body should not be disturbed for at least 8 hours?

I read that buddhists believe that once a person dies their body should not be moved or touched for atleast 8 hours because it could be "painful". I understand that the mind goes through its different phases before its ultimately reincarnated but how might it be "painful". Do the different mental stages we go through get disturbed? Living in the u.s it's going to be hard to leave a body to sit for 8 hours with autopsys and the whole process of embalming the body and everything.