Edna Graham, of Christchurch, asks :-
Why don't we sneeze when asleep?
Robin Taylor, a resporatory medical specialist at the University of Otago, responded.
In truth I do not know! The purpose of sneezing is to clear the upper airway, and it is a reflex which is present at birth.
When asleep, many sensory inputs to the brain are modified or even shut down, and this is exemplified in the fact that despite some pain somewhere, a person may still fall asleep. Breathing is modified significantly during sleep, and it becomes more regular. This coincides with a variety of other physiological mechanisms including heart rate and urinary output. For whatever reason there is an "override" as far as some of the protective upper airway functions are concerned, and they are no longer primed, perhaps because the individual will not be speaking or swallowing while asleep.
Sneezing is in this category but why this is advantageous is difficult to judge. I am sorry I can't be more helpful.