Date: 202004
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Light: Daylight
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Takeoff / Launch
Airspace: Class B ZZZ
ATC / advisory: Tower ZZZ
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Distraction; Situational Awareness; Communication Breakdown
Second reporter function: First Officer; Pilot Flying
Anomaly: ATC Issue All Types; Deviation / Discrepancy - Procedural Clearance; Flight Deck / Cabin / Aircraft Event Other / Unknown
Detector: Person Flight Crew
When detected: In-flight
Result: General None Reported / Taken
Contributing factors: Human Factors; Procedure; Environment - Non Weather Related; Company Policy
Primary problem: Human Factors

Narrative: I am submitting this report to address two things - 1) My crew accepting a takeoff clearance; when ATC called us by the wrong callsign; and 2) A general lack of alertness that I am seeing on the line as a result of these extremely irregular times.Regarding the takeoff clearance - we were Number 1 at the hold short for runway at ZZZ. We were the only aircraft out there. Nobody else in line or even within 300 yards of the runway. ATC issued a takeoff clearance for runway and the callsign was very similar to ours. My FO accepted the clearance (reading back the correct callsign) and off we went. In retrospect I believe that ATC referred to us as Company XXX (not YXX). This became clear to me when he said it again and corrected himself while switching us to departure. In truth there was no doubt in my mind that the clearance was intended for us because we were literally the only ones there; but I still should have clarified it. The takeoff was uneventful. I believe that a factor in this event is the fact that all of us are becoming a little too complacent with [few] people on each flight and virtually no other traffic around us. Every flight is starting to feel like that delayed 2 AM departure where the usual tension of flying aircraft tends to go away with direct clearances that never happen and comms that are a bit too relaxed.  We are letting our guard down at a time where we can ill afford it.I should have stopped and asked the Controller to clarify. Even though I was 100% sure that the clearance was intended for us; I let my guard down. Verify that clearance was for Company YXX. He says oops sorry; we read it back; and off we go. If we weren't the only ones there or the field was more complicated or congested that could have bitten us. As a group we need to re-dedicate ourselves to being careful and methodical in our flying. Our airline and our industry are already hanging on by a thread.  I can't imagine a worse time to add an accident to the mix.  Nothing changes just because the aircraft or the airport or the sky are virtually empty.  We need to do things the right way - the professional way. Anything less leaves our Company and our futures to chance.


Second narrative: The Captain told me the next day he filled out a report about ZZZ Tower using the wrong call sign for us to take the runway and depart. I am not sure if that happened; but I have no reason to doubt that it did.  Our callsign was YXX and the Captain said he mixed up the numbers. I didn't catch it. I read back my callsign and we took the runway and departed. At the time we were the only aircraft at the runway ready for departure.I could listen up a bit closer to catch those moments of dyslexia that we all have from time to time and query the controllers. Also; the other crew member could speak up when hearing something differently.


Synopsis: Flight crew reported a similar callsign issue resulting in a takeoff without clearance.  The crew attributed this to a lack of alertness and complacency due to the irregular times.