Light Transport; Low Wing; 2 Turboprop Eng
Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor; Flight Crew Multiengine
Flight Crew Last 90 Days 150; Flight Crew Total 11030; Flight Crew Type 1825
Fatigue
Conflict Airborne Conflict; Deviation - Altitude Overshoot; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Provided Assistance; Flight Crew Returned To Clearance; Flight Crew Took Evasive Action; Flight Crew Became Reoriented
Staffing; Company Policy; Human Factors
After driving 6 hours and being fatigued from flying 32 legs the previous week; I deviated from my assigned altitude. The controller pointed out the oncoming traffic; which I even altered course 15 degrees to avoid the oncoming aircraft. I suddenly discovered that I exceeded my assigned altitude of 12000 feet by at least 500 feet. I rapidly made a descent back down to 12000 feet. Visual contact with the oncoming aircraft was never lost until passing and my main goal was to maintain visual separation with that traffic.I would be remiss if I didn't say that fatigue and emotional factors contributed greatly to this pilot deviation. My company is short on pilots and long on work. I was covering this run for the time being until we hire a pilot to fly it permanently. Just another example that even if you are getting legal crew rest doesn't mean it's safe.
Pilot drifted above cleared altitude conflicting with opposite direction traffic.  The pilot reporter corrected the error after Controller advised him of the situation. Pilot commented fatigue was a contributing factor.