Shortness of breath while lying down or that occurs after several hours of sleep can be a sign of heart failure. Shortness of breath may be a result of buildup of fluid in the lungs (pulmonary edema).

When a person is standing or sitting upright, gravity helps keep the lung area relatively free of excess fluid. The effect of gravity is lost and fluid can easily build up in the lungs when a person lies down.

In heart failure, the heart is not able to handle the increased volume of returning blood. When lying down, fluid from the backed-up blood collects in the lungs, causing shortness of breath or a feeling of suffocation.