People with orthopnea have trouble breathing when they're lying down. It's a telltale sign of heart failure.

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Counting your pillows, not sheep, as you go to sleep at night might reveal a telltale sign of a serious heart condition.

People with some types of heart or lung problems often complain about difficulty breathing while lying down. For many with this condition, this trouble breathing or sensation of breathlessness is relieved only by sitting up straight or standing. Sometimes a person realizes that sleep is more comfortable when propped up with extra pillows under his or her head. This condition is called orthopnea, and it's commonly measured by the number of pillows needed to ease the breathing ("three-pillow orthopnea, for example").

The word orthopnea comes from the Greek terms "orthos," which means "standing upright or straight," and "pnoe," which means "breathing."

It's important to report orthopnea to your doctor, even if you can control it by simply propping your head up while you sleep -- it may be a sign of heart failure danger. One study has shown that heart failure patients with persistent orthopnea have poorer prognoses and are at significantly higher risk of hospitalization than patients without orthopnea. The study concluded that this group of patients requires a more aggressive approach to improve their outcome.­­