What causes shortness of breath when lying down and coughing at night, but not during the day?

Shortness of breath when lying down and coughing at night, but not during the day is a classic symptom of heart failure, known as orthopnea. Heart failure occurs when the heart can no longer pump an adequate amount of blood to meet your body’s needs. When you are lying flat in bed at night, gravity causes more fluid to pool in your lungs, making it harder for your already weakened heart to push enough oxygenated blood through your body. This results in shortness of breath even with just normal activities like coughing or talking. Other symptoms may include fatigue and swelling in the ankles or legs after standing for long periods of time during the day and sleeping pillows may need to be propped up so that you sleep at an incline, which helps reduce fluid buildup in the lungs while sleeping. If you have these symptoms it's important to get them checked out by a doctor as they could potentially be signs of much more serious issues such as coronary artery disease or asthma related complications from GERD (gastroesophageal reflux disease).