Having the occasional stuffy/runny nose is the closest that I get to being sick, and whenever it happens I always have the same observation.
I am only stuffed up on one side of my face while the other side is crystal clear. To which then I notice that a couple of hours later, it’s switched on me! Is there a physical reason for this? Or is this a case of an illusion created by my senses?
There is a definite physical reason for it. You may be surprised to learn that you have erectile tissue in your nose:
For some reason, the degree of engorgement of the turbinates cycles from side to side every few hours. You only notice this if you are congested, because otherwise the engorged side is still open.
It can also switch at night if you sleep on one side versus the other due to blood pressure. The side closest to the ground may get more congested due to engorgement of blood in the turbinates.
Almost everyone has different airflow between one nostril and the other, and it tends to switch every few hours. You typically only really notice it when you’re congested, but for most of us, it’s always there, and it apparently has to do with optimizing our sense of smell. It seems that our ability to detect smells and judge them depends on the airflow, so the differential gives us greater sensitivity and discrimination.