Ongoing, high levels of negative stress may directly and indirectly increase the danger of developing heart disease, depending largely on individual responses to that stress. For example, a person who reacts to environmental stress by experiencing increased physical and mental tension, chronic anxiety, irritability, or obsessive behaviors, may also experience accompanying heart strain, palpitations, and continuously high levels of adrenaline. Over time, these physical conditions can increase the risk of cardiovascular disease. However, the clearest relationship between stress and cardiac health lies in the indirect ways stress affects an individual. Many common responses to stress involve behaviors that often contribute to the risk for heart problems. For example, chronic or overwhelming stress may lead a person to take part in unhealthy activities such as drinking alcohol, smoking cigarettes, or overeating. In other cases, the reaction to heavy, ongoing stress may be clinical depression and an accompanying lack of physical activity. In addition, habitual, excessive drinking of alcohol can cause irregular heartbeats, contribute to obesity, and increase blood cholesterol and blood pressure, all of which are key risk factors for heart disease. Cigarette smoking doubles the possibility of a heart attack due to its many adverse physical effects. Smoking can raise levels of bad cholesterol, making coronary artery disease more likely. It also can reduce the lung's efficiency of oxygen exchange, forcing the heart to work harder to pump sufficient oxygen to the body. Over time, the extra work may cause an enlarged heart, high blood pressure, and even heart failure. As a response to stress, overeating can easily lead to excessive weight gain or obesity, making heart disease and stroke more likely as well. Such excess weight usually puts heavy strain on the heart, raises blood pressure and cholesterol levels, and increases the risk for diabetes, all of which may directly increase the chances for developing coronary heart disease. Finally, depression and the accompanying lack of physical activity may allow the heart to become de-conditioned and less able to handle the strain of normal physical exertion.