Date: 202009
Local time of day: 1201-1800
Location: ZFW.ARTCC, TX
Light: Daylight
Aircraft: B777-200
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZFW
ATC / advisory: Center ZFW
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Workload; Fatigue; Human-Machine Interface
Second reporter function: Relief Pilot; Pilot Not Flying
Anomaly: Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural FAR
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Returned To Clearance; Flight Crew Overcame Equipment Problem; Flight Crew Became Reoriented
Contributing factors: Human Factors
Primary problem: Human Factors

Narrative: We were in a very busy communication with Dispatch about the ZZZ weather and possible divert plan. We were receiving dozens of ACARS pages of info from Dispatch. ATC Center; I think; gave us a decent from FL400 to FL390. I put it in the MCP Altitude window and the FO (First Officer) confirmed. I gave him the aircraft and proceeded to call dispatch. Somehow; FL380 was put in the window. The ATC controller called to confirm we were supposed to level at FL380. We bottomed out at about FL382 and promptly returned to FL390.The cause was we were extremely busy dealing with dispatch; weather and route changes and then transferring the aircraft to the FO; who had just minutes earlier returned from his rest break. All of us are rusty with the reduced flying. Wake up call is [early] in ZZZZ. Fatigue at the end of a [long] flight has everyone a bit off their game. Add in not flying much.Returning at the end of a long flight we failed to catch an otherwise normally caught error. [We should] refocus on Pilot Flying and confirming the altitude.


Second narrative: I had just come off break. When I sat down there were over 10 messages waiting on the com page and I was told Dispatch wanted us to divert. We were in VERY busy communication with Dispatch about the ZZZ weather and possible divert plan. We were receiving dozens of ACARS pages of info from Dispatch. ATC Center gave us a decent from FL400 to FL390. The Captain put it in the MCP Altitude window and I confirmed it. He gave me the aircraft and proceeded to talk to Dispatch. Somehow; FL380 was put in the window. The ATC controller called to confirm we were supposed to level at FL380. I immediately noticed the error; put FL390 in the window and use vertical speed to arrest the descent and start a climb back up to FL390. We bottomed out at about FL382 and promptly returned to FL390.The cause was we were extremely busy dealing with Dispatch; weather and route changes and then transferring the aircraft to me was probably not as clear as I remember; I had just minutes earlier returned from my rest break. The altitude change occurred immediately after I sat down to this hectic environment. All of us are rusty with the reduced flying. Fatigue at the end of a long flight has everyone a bit off their game. Add in not flying much.When returning at the end of a long flight we failed to catch an otherwise normally caught error. [We should] refocus on Pilot Flying and confirming the altitude. Have the FB more in on the process on the initial seat swap.


Synopsis: B777-200 flight crew reported an altitude deviation during cruise due to fatigue.