Why Don't We Sneeze When We Are Asleep?

Sneezing, is as stated in the previous post, is a reflex action in response to stimulus in the nasal passage to the brain.  During sleep, most of the activity of the brain declines we are less responsive to external stimuli and the reflex action is not initiated. As such sneezing does not take place during sleep. But if the stimulus is very intense, it can waken the brain and sneezing will occur and you will wake up.

Why Do We Sneeze?

Sneezing is a procedure by which foreign particles, harmful bacteria, viruses etc. that have accumulated in your nasal passage due to a cold or allergic reactions or the blowing of dust in your face are expelled and is a reflex action - an action that is done automatically over which you do not have any control. When there are foreign material in your nose a chemical called histamine is produced which sends signals to the brain that there are material that should not be there. The brain now sends out a message to the lungs to squeeze them and also shuts down the pharynx to prevent air from being squeezed through the mouth. The nose expels these foreign material with a blast of air that may at times reach speeds of some 950 km/hr.