Cardiac syndrome X consists of a triad of chest pain, abnormal exercise stress testing and normal coronary angiogram, and is hypothesized to be related to endothelial dysfunction. Endothelial dysfunction is also reported to be linked to obstructive sleep apnea. While chest pain can be one of potential presenting symptoms of obstructive sleep apnea, the investigators hypothesize that obstructive sleep apnea is common in subjects with cardiac syndrome X.