why can i only breathe through one nostril

It is normal for people to breathe predominantly through one nostril at a time due to a natural process called the nasal cycle, where the blood vessels in one nostril swell and narrow the air passage while the other nostril's passage opens wider. This cycle alternates every few hours to regulate airflow. However, if breathing is consistently difficult through one nostril, it may be due to issues such as nasal congestion from allergies, colds, a deviated septum (where the wall between the nostrils is off-center), nasal polyps, or injuries that block the nasal passage. Persistent one-sided nasal blockage or difficulty breathing may require medical evaluation. Also, factors like sleep position can influence which nostril feels more congested at a given time.