The nose is separated into two passages by a wall of cartilage called the septum. The nasal passages are lined with a membrane that produces a clear liquid called mucus. Mucus is a one of the body's defense systems:
The mucus traps small particles and bacteria, which may enter the nose as a person breathes.
The trapped bacteria usually do not cause harm in healthy individuals.
However, the bacteria can lead to a daily cycle of congestion and decongestion.
When one side of the nose is congested, air passes through the open (decongested) side. The sides alternate between being wide-open and partly or completely blocked.