A man was critically injured Monday morning when he fell asleep at the wheel and slammed into a tractor on Sparta Highway.The motorist, Kelly D. Cash, 44, of Sparta was airlifted from the scene to Vanderbilt University Medical Center by LifeFlight helicopter for severe arm and leg injuries.According to highway patrolman Rodney Whiles, the accident happened just before 9 a.m. on Sparta Highway near its intersection with Highway 30 close to the flashing caution light.Cash was traveling westbound, toward McMinnville, on the four-lane in his 1992 GMC Sierra when he reportedly fell asleep. His vehicle then overtook a John Deere tractor which was also westbound, slamming into the rear of the moving farm equipment. The impact crushed the driver’s side of the pickup, which then ran onto the shoulder of the highway.The driver of the tractor, Farrell C. Rogers, 64, of Rock Island was taken to River Park Hospital for treatment of injuries.