Obstructive sleep apnea (OSA) is characterized by repeated episodes of apnea and hypopnea during sleep. Obesity is the most important risk factor for OSA. From the recent reports, the prevalence of OSA is high. Data have shown that severe OSA (the mean number of apnea and hypopnea episodes per hour slept: AHI > or = 30) is a risk factor for cardiovascular diseases and death. In addition, OSA is widely prevalent in patients with obesity, diabetes, and hypertension which are the major risk factors for cardiovascular disease. Several reports show that sleep duration has significant effects on BMI, mortality and diabetes. It has been reported that leptin and ghrelin, which were the two key hormones in appetite, are elevated in OSA patients. Therefore, it is important to investigate the relationship between OSA, body weight gain and obesity.