The average person spends 1/3rd of his or her life sleeping. It’s quite a sizable portion of our lifetime. So why is it that we never sneeze while we’re unconscious? Or do we? Today we explore this little life mystery.

What Is A Sneeze?

A sneeze is a reflexive action usually produced by your body in response to an irritant in the nasal mucosa. However, all reflexes have thresholds.

Similar to the knee jerk when the doctor hits it with a little rubber hammer, we sneeze when dust or other irritants stimulate specific areas of nasal tissue. When those areas inside your nasal cavity and sinuses are stimulated, a signal is then sent to your brain. The brain receives the message and sends a signal of its own in response which travels through our central nervous system to tell our face, throat, and chest muscles that it’s time to sneeze.

Why Don’t We Sneeze In Our Sleep?

As we fall asleep, the body relaxes parts of our brain which is responsible for reacting to certain stimuli. One of these parts is the part that triggers the reflex for sneezing. The part responsible isn’t completely shut-off however. It simply causes the reflex threshold which tells your body to sneeze to be raised. This means that it requires much more stimulation to initiate a sneeze. This is thanks to an inhibitory neurotransmitter known as GABA, which specifically inhibits certain areas of the brain that are more active when we’re awake.

While it is possible to receive enough stimulation to sneeze while you are asleep, the amount required would wake you up long before you could actually sneeze.

Why Do Bright Lights Sometimes Cause Us To Sneeze?

Sometimes a strong enough light source is enough to cause a person to sneeze. This is called a photic sneezing reflex and affects roughly 18–35% of the population. Scientists are not sure exactly how or why this happens, but the leading theory is a few crossed wires in the brain.

The trigeminal nerve, a nerve responsible for facial sensation and motor control, is right next to your optic nerve. When exposed to a bright light, like the sun, the optic nerve sends a signal the brain to constrict the pupils. When this happens, a portion of the electrical signal is sensed by the trigeminal nerve and mistaken by the brain as an irritant in the nose. The brain then tells your body to sneeze.

Bonus Fact: The myth that “you can’t sneeze with your eyes open because the force will cause your eyeballs to pop out” is false and is shown debunked below in an episode of Mythbusters.





Photos are available under a Creative Commons Attribution license by Wikicommons.

References:

“A Moment of Science: Sleep On, Sneeze Not“. Retrieved 2008-11-14.

“Solar Sneeze Reflex“. Western Journal of Medicine 146 (5): 20. 1 May 1987. PMC 1307391.

Watanabe M, Maemura K, Kanbara K, Tamayama T, Hayasaki H (2002). “GABA and GABA receptors in the central nervous system and other organs”. Int. Rev. Cytol.. International Review of Cytology 213: 1–47.