Prompted by the observation that a fellow medical student sneezed at about the same time in class every morning, we recorded the time of each of the 118 sneezes she made on 69 days over a 6.5-month period. Analysis of the sneeze times with the goodness-of-fit test revealed a highly nonuniform distribution (p < 0.0001) with a significantly increased probability of sneeze production within a small time interval centered at 8:20 AM. After ruling out the known causes of sneezing, we propose that the subject had a circadian fluctuation in her sneeze threshold independent of any immediate external stimulus. We compare this circadian rhythm to other periodic phenomena such as cyclic esotropia and rapid-cycling bipolar disease as well as to illnesses such as the periodic paralyses in which rapid and reversible changes occur in the neuromuscular system, and we discuss the peculiar circumstances favorable to documenting a circadian sneeze.