Background The purpose of this study was to investigate the changes in sleep characteristics in patients who underwent coronary artery bypass grafting (CABG) surgery, using both subjective and objective tests in the early (preoperative) and late (postoperative) periods. Methods and Results Forty-five patients who underwent CABG and did not previously have any sleep disturbance were evaluated by subjective and objective sleep parameters during a consecutive 3-5-day preoperative examination, during a consecutive 5-8-day period in the 1^st postoperative week, and during consecutive 5-8-day periods in the 1^st and 2^nd postoperative months. The Pittsburgh Index and Epworth Sleepiness Scale values, sleep latency, napping episodes, total napping period, duration of wakefulness after sleep onset and fragmentation index values were significantly increased; however, Maintenance of Wakefulness Test lengths, total sleep time and sleep efficiency were significantly decreased in the 1^st postoperative week. All of these were the same in the 1^st postoperative month and differences were not statistically different from the preoperative period. None of the sleep parameters in the 2^nd postoperative month differed from the values obtained in the preoperative period. Conclusion The cause of sleeplessness after CABG surgery may be temporary deterioration of circulation in the centers of the brain stem and hypothalamus that control sleep and awakening. Improvement of the circulation in these centers a few months after the operation helps to regain sleep control, and thus sleep disturbances disappear. (Circ J 2007; 71: 1506 - 1510)