Primitive women, before man invented the fire, slept on the dusty hard ground, either in shallow caves or rock shelters, or beneath the open sky. They slept, outside of the time they were busy servicing the male, from sundown to sun up. They woke up to the bright rays of the morning sun and sneezed to expell the dust and attendant germs that they had breathed in during the night. Sneezing on rainy days wasn't as important because the high humidity keep the dust down.

But now I wonder...why is sundown one word and sun up two?