When you go outside and bright sunlight hits your eyes, do you sneeze? In other words, do you have a photic sneeze reflex?
I sure do. I've been reminded of it frequently during the past few weeks. Spring has arrived on the calendar, and the weather in Vermont is starting to get a clue. A sunny day in the 60s (Fahrenheit) can make you feel so good after a long winter. So often I make a point of escaping my office for a moment, just to take it all in. And sure enough, if it's so bright that I can barely keep my eyes open, I instantly feel that little tickle in my nose, and... Gesundheit! Works every time.
So what's up with that? Why should people sneeze in bright light? It's tempting to come up with a useful function for the photic sneeze reflex; I mean, it couldn't be pointless, could it? For example, maybe the reflex is a mechanism for protecting your eyes. Potentially harmful light rays hit the retina, and the brain forces you to shut your eyes by making you sneeze. Seems like a Rube Goldbergian way to force you to shut your eyes, but why not? The reflex appears to be more common in kids, who would presumably need it more because, compared to adults, they are less likely to have learned that staring at the sun is bad.
Then again, maybe staring at the sun isn't quite as bad as popularly believed. But even if it is, we already have a more direct reflex - called the dazzle reflex - that closes the eyes automatically in blindingly bright light, no sneezing required. Most people don't even have the photic sneeze reflex (although it's not clear exactly how common it is - more on that point below). And according to the generally trustworthy Snopes.com, it is possible for some people to keep their eyes open when they sneeze (no, I'm not one of them).
A larger issue is that reflexes don't need to have a "purpose." There are over 200 reflexes listed in Dorland's Medical Dictionary, and many of them, such as the rooting reflex and sucking reflex in infants, make sense from a functional perspective. But some reflexes really don't. Take the cremasteric reflex. When the upper inner thigh of a man is stroked gently in a downward direction, the scrotum contracts, elevating the testes. It's a cool trick, and one that happens to be clinically useful because its absence can confirm conditions such as testicular torsion or damage at upper lumbar levels of the spinal cord, to name a few. But you'd have a hard time convincing me that the cremasteric reflex has an adaptive function. It's just a consequence of the way things are wired.
Whether or not it has a purpose, the photic sneeze reflex is a real phenomenon that has actually received some attention in the medical literature (although I confess that I first read about the reflex years ago in a hilarious Straight Dope article ). The best scholarly reviews I've found were published in 1964 and 1993, both in the prestigious journal Neurology. (Unfortunately the articles are both too old to be online - you'll have to scope out the nearest university library to find them.) The reflex appears to be inherited as an autosomal dominant condition, which means that if Mom or Dad has it, each of their children has a 50/50 chance of getting it too. Geneticists with a sense of humor have even given it a special name: the ACHOO syndrome (Autosomal dominant Compelling Helio-Ophthalmic Outburst).
How common is the reflex? The usual answer is 25%, but studies so far have relied on small sample sizes and/or homogeneous populations and the results vary considerably (from 11-36%). No one has really done the "definitive" photic sneeze reflex survey.
So, mostly for fun, I decided to do my own online survey at the Student Doctor Network. As of today, there have been 239 respondents. Obviously it's not a scientific survey, but the SDN forums are designed so that only registered users can vote, and each user can vote only once. It's technically possible for one person to vote multiple times if he/she has multiple user names, but why would anyone bother? It's also possible that some respondents lied, but users are generally anonymous, and in this case there isn't any obvious incentive to be dishonest. Nothing is at stake.
Below are the results, which you can also see here:
Do you sneeze when exposed suddenly to bright light?
Yes, all the time. 20.1% Yes, sometimes. 27.6% Yes, but only rarely. 8.0% No, never. 23.9% Is this a joke? 20.5%
Probably the most surprising outcome for me is the high percentage - nearly half - of respondents who sneeze at least "sometimes" when exposed to bright light. That's double the usually cited frequency. However, it's not much higher than the 36% reported in a small study of Baltimore neurologists. Probably depends on how you define a "photic sneezer." Is sneezing "sometimes" in the sunlight good enough? Previous studies have also shown evidence of variation with factors such as age and race.
I actually wasn't surprised that so many respondents went with, "Is this a joke?" That phenomenon was described first in the 1964 review:
Finally, it is interesting to note some attitudes of those being questioned about the photic sneeze reflex. Many who do not have it have reacted to such questioning a surprised way, as much as to say, "Are you crazy or something?" Conversely, a woman who does sneeze to light reacted by saying, "Why, I though everyone did!"
Here are a couple other things I learned from the SDN survey:
Respondents mentioned a number of other "sneeze stimulants," including mints, grapefruit, chocolate, red wine, and plucking eyebrows. As bizarre as it sounds, plucking eyebrows (along with mints and wine) was mentioned specifically in the 1993 review as a known trigger of sneezing.
Several respondents cited the practice of using light to push an impending sneeze "over the edge." One respondent even trained herself to sneeze:
I have actually trained myself to sneeze on demand. I know this sounds odd, but once I figured out that sunlight made me sneeze, I began looking up towards the sun or another bright light anytime I felt like I was about to sneeze. I did this in order to speed up the time between when I first felt a tickle to when I actually sneezed.
Now, if I am thinking about sneezing, and I look up, I will sneeze. Even if it is in a dim room.
I know I am odd. It's okay.
Well, odd or not, I think sneezing on demand is pretty cool!