It's not uncommon to experience harmless episodes of breathlessness for brief periods. These spells can be caused by heavy exercise, sudden physical exertion, momentary anxiety, or surprise. Persistent anemia or thyroid disorders can also upset the body's oxygen supply and demand. However, ongoing breathing difficulties may also be a symptom of more complicated underlying physical problems, such as heart or lung disease. In congestive (kon-JESS-tiv) heart failure, for example, it becomes difficult to catch a breath because fluid enters the lungs air spaces and creates pulmonary congestion or pulmonary edema (ah-DEEM-ah). At first, shortness of breath occurs only during physical effort. However, as the heart failure progresses, it may become more and more difficult to breathe normally even during rest periods. In fact, shortness of breath may become most noticeable in a reclining position, since this makes it easier for fluid to leak into and move throughout the lung tissue. If the condition improves by sitting up, it's termed nocturnal dyspnea (nok-TUR-nall DISP-nee-ah), due to the aggravation of nighttime sleep. Aside from this relationship to heart disease, breathlessness may also be a symptom of lung disease, respiratory muscle problems, or a nervous system disorder. If you're concerned about shortness of breath or have questions about heart disease, contact a healthcare provider.