Sneezing at same time every night

Squeek has started sneezing around 1:30 am every night. We can almost set our clocks by it. Its usually 2 really big sneezes in close succession, you can see her making the 'about to sneeze' face and them 'achoo!...pause, make the face again, 'achoo!'. Can't think of whats trigging this at the same time every night. Its happened in different rooms on different nights. Could it be an allergy?