Nasal cycle

The nasal cycle is the alternating partial nasal congestion and decongestion of the nose cavities. It occurs in humans and other mammals. The individual is usually not aware that it happens. 
It results in greater airflow through one nostril, and regular alternation between the nostrils. The hypothalamus selectively activates each half of the autonomic nervous system in turn. The phenomenon is quite normal.
What happens.
The cilia of the congested side suspend their motility until that side decongests. The cycle means that, at any time, one side of the nose is moist, and moistens the incoming air. This is one of the three functions of the nose. The other two are the filtration and warming of fresh air before it gets to the lungs.
It is not entirely clear what the benefit is of this system.