Sleep-disordered breathing including obstructive sleep apnea (OSA) is an extremely common medical disorder associated with important morbidity. The purpose of this study is to understand the clinical features and course of patients with sleep apnea in Korea. Participants who are suspected of sleep apnea due to symptoms such as snoring, witnessed apneas, waking up with a choking sensation, excessive sleepiness, non-restorative sleep, difficulty initiating or maintaining sleep, fatigue or tiredness, and morning headache will be prospectively recruited at the outpatient clinic of Seoul National University Hospital. After polysomnography (PSG) or respiratory polygraphy, patients with sleep apnea including OSA and central sleep apnea (CSA) will be followed regularly. Participants not having sleep apnea (apnea-hypopnea index \< 5/hour) will only have a baseline visit. For only participants with the informed consent of donating blood, their blood samples will be collected and stored in the Seoul National University Hospital-Human Biobank (SNUH-HUB).