It is well known that symptoms of chronic lung diseases including asthma and COPD worsen depending on the time of day, which has sparked interest in understanding how the circadian clock regulates other chronic lung complications including cystic fibrosis (CF), pulmonary arterial hypertension (PAH), and idiopathic pulmonary fibrosis (IPF) [9]. This evidence concerns the gene CLOCK and pulmonary arterial hypertension.