Insomnia is common in many patients with chronic obstructive pulmonary disease (COPD). The causes of insomnia are sleep induced pathophysiological effect of COPD itself, COPD comorbidities and the presence of coexisted obstructive sleep apnea. Sleep has profound adverse effects on respiration and gas exchange in patients with COPD. There are several mechanisms underlying oxygen desaturation during sleep. They include decreased functional residual capacity, decreased ventilatory responses to hypoxia and hypercapnia, impaired respiratory mechanical effectiveness, respiratory muscle fatigue, decreased respiratory drive, and increased upper airway resistance. COPD comorbidities include DM, cardiovascular diseases, osteoporosis, depression, and GERD. The coexistence of COPD and sleep apnea-hypopnea syndrome has been denominated "overlap syndrome".