Commercial Fixed Wing
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Fatigue; Situational Awareness; Other / Unknown; Distraction
Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Other / Unknown
Flight Crew Took Evasive Action
Procedure; Human Factors; Environment - Non Weather Related
During the flight preflight; I missed resetting the flap selector to 10 degrees; as is my normal practice.  During taxi out and completing the before takeoff checklist; the challenge question was asked 'Flaps 10;' and due to expectation bias; and monitoring an aircraft back taxiing on the runway; I saw and confirmed 10 degrees of flaps; instead of the 15 degrees actually dialed in. When Tower cleared us for takeoff I advanced the throttles; and the aircraft horn sounded due to the incorrect flap setting; I pulled the throttles to idle at approximately 5 knots applied brakes; then we reset the flaps to 10 degrees; and after confirming the correct configuration continued the takeoff.2nd leg of the day; a day after an oceanic crossing; followed by isolating in the hotel.  This accompanied by the COVID-19 restrictions caused broken sleep periods. While I felt fine; the fact that I was only getting 3 1/2 to 4 hours of sleep in blocks; coupled with the circadian disruptions did not help.Better attention to detail.
On initial power push for takeoff; received the flap warning horn.  The Captain brought throttles back; I reset the flaps to 10 and we continued the takeoff roll.  I failed to check the flap setting when we input the numbers and again when I  we accomplished the before takeoff checklist.  A recheck when I called 'Configuration Checked' may have caught it; however the dark most likely would have prevented this from happening.  Do not assume and be sure to verify settings called.  I failed to do this and allowed myself to get in a hurry.
Air carrier flight crew reported receiving a flap setting warning horn during initial power push for takeoff. The Captain cited lack of sleep as a contributing factor.