circadian rhythms

This is a blog post that I more or less composed in my mind early this morning when, at 5 AM, I awoke with a runny nose and started sneezing. Just like I have every morning at 5 AM for the past month and a half. I do not, as far as I can tell, have a cold. I do not experience the same runny nose or sneezing at other times of day. Is it possible that I'm allergic to 5 AM? (People can't actually be allergic to a time of day, can they?) Or, more plausibly (if only marginally so), is it possible that something about my circadian rhythms makes me more vulnerable to allergens in my environment…