Go! flight 1002 departed for its destination about 0916 Hawaii standard time. About 0930, the captain transmitted to air traffic control (ATC) that the flight was climbing through 11,700 feet to its cruise altitude of flight level (FL) 210 (approximately 21,000 feet mean sea level.) The controller acknowledged the transmission and cleared the flight to proceed to an intersection along the flight route located about 29 miles north-northwest of the destination airport, and the flight crew acknowledged. At 0933, during cruise flight, the controller repeated the navigational clearance, which the flight crew acknowledged, and the airplane turned in accordance with the clearance. At 0940, the controller instructed the flight to change radio frequencies, and there was no response. The controller continued to try to contact the flight crew multiple times but received no reply. At 0951, the airplane crossed the intersection that was its clearance limit then turned southeast toward the destination airport without descending, which is consistent with the airplane being on autopilot. The controller handling the flight asked another controller to attempt to contact the flight crew on a different frequency, but there was still no response, and the flight proceeded on a southeasterly heading at FL 210. About 0955, the flight crossed over the destination airport and continued on a southeasterly course without changing altitude or heading. Two separate airline crews in the area attempted to contact the incident crew, but neither flight crews? attempts were successful. About 0958, when the flight was about 26 nautical miles southeast of the destination airport, the captain contacted the controller with an abbreviated call sign (?Ah HCF ten zero two?), and the controller asked if the flight crew was experiencing an emergency. The captain responded, “No, we must have missed a hand off or missed a call or something.” The controller then issued instructions for the flight to return to the destination airport, with which the flight crew complied. The flight arrived without further incident about 1015. The captain and first officer both reported to their company that they had unintentionally fallen asleep in flight. The fact that both pilots fell asleep during the midmorning hours, a time of day normally associated with wakefulness and rising alertness, indicates that both pilots were fatigued. The captain had undiagnosed severe obstructive sleep apnea, which was diagnosed during a medical evaluation shortly after this incident and for which symptoms (such as snoring) and risk factors (such as obesity) were present before the incident. This condition likely caused him to experience chronic daytime fatigue and contributed to his falling asleep during the incident flight. Probable Cause: The captain and first officer inadvertently falling asleep during the cruise phase of flight. Contributing to the incident were the captain’s undiagnosed obstructive sleep apnea and the flight crew?s recent work schedules, which included several consecutive days of early-morning start times.