Difficulty in breathing while sleeping on the left side may be caused by diseases such as right pleural effusion, right pneumothorax, and soft tissue prolapse of the upper airway. 1. Right pleural effusion: refers to excessive accumulation of fluid in the right pleural cavity. When the amount of fluid is small, the symptoms are not obvious; when there is a large amount of fluid, there can be palpitations, dyspnea and other symptoms. When sleeping on the left side, the effusion compresses the left side of the lungs, which weakens the compensatory function of the left side, thus aggravating the dyspnea. 2. Right-sided pneumothorax: refers to excessive gas in the right pleural cavity. When the amount of gas is small, the symptoms are not obvious; when a large amount of gas is present, there may be symptoms such as dyspnea. When sleeping on the left side, the gas compresses the left side of the lungs, which weakens the compensatory function of the left side, thus aggravating the dyspnea. 3. Upper respiratory tract soft tissue prolapse: if the upper respiratory tract soft tissue prolapse, prolapse is obvious when sleeping on the left side, can block the respiratory tract, thus appearing the left side of the dyspnea when sleeping. There are many other causes of dyspnea when sleeping on the left side, so we should pay attention to it, consult the doctor in time to find out the cause of the disease, and follow the doctor’s advice for further treatment or therapy.