An Air Carrier First Officer noticed that the Captain was incoherent and unresponsive. He declared an emergency and landed at the nearest airport. Diabetes-related loss of consciousness is suspected to be the cause.

Narrative

After reaching cruising altitude; about thirty minutes into the flight; I noticed that the Captain's behavior had changed. He became incoherent and unresponsive to questions. I then noticed that he began to make fists with both hands; shake; and bite his tongue. I continued to ask him if was okay. After I received no response I alerted the Flight Attendant to a possible seizure with the Captain and asked her if she would help with assessing the situation while I flew the plane. I then declared an emergency with Center and requested to land at the nearest airport. We landed 2;000lbs overweight due to the emergency and the unscheduled landing. The Captain remained non-responsive and in the same condition throughout the decent and landing. Once on the ground; I brought the aircraft to a stop on the runway and shut down the engines. Emergency crews met the aircraft and removed the Captain to an awaiting ambulance. At this time the only conclusion I can come to is that the Captain has continued issues with diabetes. This was not known by the rest of the flight crew. From updates received while waiting for a replacement Captain; we were informed he had suffered from issues related to his disease. A stronger review of medical issues that an individual has must be addressed in this type of event. We cannot allow a person who has continued medical problems to fly until they are proven to be healthy and fit for flight.