One nostril becoming clogged when a person lies on their side is a common experience and is often due to the structure of the nasal passages. The nasal passages are divided into two sides by a thin wall of cartilage called the septum. The septum has a small opening, called the nostril valve, which allows air to flow between the two sides of the nose.

When a person lies on their side, the force of gravity causes the contents of the nasal passages to shift. For example, if a person lies on their right side, the contents of the left nasal passage will shift towards the nostril valve, potentially blocking it. As a result, the person will experience difficulty breathing through their left nostril, and it may feel clogged.

In some cases, a person may have a deviated septum, which can make them more prone to experiencing a clogged nostril when they lie on their side. A deviated septum is a condition in which the septum is curved or tilted, resulting in a blockage of the nasal passages. A person with a deviated septum may experience difficulty breathing through one nostril even when they are not lying on their side.

Additionally, certain medical conditions can cause one nostril to become clogged when a person lies on their side. For example, sinusitis is a common condition that can cause inflammation and swelling of the nasal passages. This can result in a feeling of congestion and a clogged nostril, particularly when the person lies on their side. Other medical conditions that can cause this sensation include allergies, nasal polyps, and a deviated septum.

In conclusion, one nostril becoming clogged when a person lies on their side is a common experience and is often due to the structure of the nasal passages. The force of gravity causes the contents of the nasal passages to shift, potentially blocking the nostril valve. A deviated septum or certain medical conditions can also cause one nostril to become clogged when a person lies on their side.