Flight Crew Instrument; Flight Crew Commercial; Flight Crew Multiengine
Flight Crew Last 90 Days 200; Flight Crew Total 1100; Flight Crew Type 784
Fatigue
Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Flight Deck / Cabin / Aircraft Event Other / Unknown; Inflight Event / Encounter CFTT / CFIT
Flight Crew Took Evasive Action; Flight Crew Became Reoriented
Human Factors
On a routine IFR flight to OME in [an aircraft] with no auto-pilot I was given a clearance to begin an initial descent from 10;000 to 7000 ft.  Somewhere in the descent I fell asleep and upon waking found our altitude was descending through 5;000.  We were in VMC at the time; but within a few miles of a mountain ridge line obscured in an overcast layer.  That same mountain ridge line caused us to lose radar contact and radio communications with Anchorage Center (normal for that route).  An immediate maximum performance climb was initiated to return to 7;000 ft.  Several minutes later radio communications were re-established and the captain reported level 7;000.  I believe the primary cause was crew fatigue.  The captain had just returned to station late the previous night following a 16 hour commute and was noticeably fatigued prior to the flight. As first officer; I had flown five of the previous seven days right to duty day maximums and was also feeling fatigued.  Our crew rest facilities are located on the airport; and a [Large Transport aircraft] came in next door around XA:00 that morning interrupting sleep for both pilots.
Cargo aircraft First Officer reported descending below cleared altitude in mountainous Alaska terrain; when he fell asleep; following a very fatiguing series of flight assignments.