WebMD notes that people do not sneeze in their sleep. Wikipedia explains that during sleep, motor neurons are inactive and reflex signals are not transmitted to the brain, which make sneezing during sleep impossible.
According to Wikipedia, sufficient external stimulants may awaken a person for the purpose of sneezing. WebMD reveals that sneezing is an immune response, which keeps the body safe and healthy by clearing foreign particles, viruses and bacteria from the nose. Sneezing begins when irritants reach the nasal mucosa and trigger a release of histamines that prompt nerve cells to send signals to the brain for activation of tracheal and pharyngeal muscles. Sneezes can travel up to 100 miles per hour and release 100,000 germs to the air.