The left side of the nostril is blocked in the morning and the right side of the nostril is blocked in the afternoon, which belongs to the alternating nostril blockage, and it may be caused by upper respiratory tract infection, chronic simple rhinitis, sinusitis and so on. 1. Upper respiratory tract infection: the upper respiratory tract is infected by pathogenic bacteria, resulting in acute inflammation of the nasal cavity caused by congestion, edema and increased secretion, which can be manifested as alternate nostril blockage. 2. Chronic simple rhinitis: chronic inflammation of the nasal cavity, causing local tissue congestion, edema, can also appear alternately blocked nostrils, accompanied by symptoms such as runny nose. 3. Sinusitis: Inflammation of the sinuses causes local tissue congestion and edema, which can lead to alternating nostril blockage, headache, decreased sense of smell and other symptoms. Nostril left morning blockage afternoon right blockage may also be other reasons, it is recommended to consult a doctor in a timely manner to identify the cause of the treatment prescribed by the doctor, so as not to miss the condition.