Date: 202012
Local time of day: 1801-2400
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb; Descent; Cruise
Reporter organization: Air Carrier
Reporter function: First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Fatigue
Anomaly: Deviation / Discrepancy - Procedural Published Material / Policy; No Specific Anomaly Occurred Unwanted Situation
Detector: Person Flight Crew
When detected: Pre-flight; In-flight
Contributing factors: Staffing; Environment - Non Weather Related; Human Factors; Company Policy
Primary problem: Company Policy

Narrative: On Day 5; the crew desk called me to assign me to a trip on Day 6 with a XA:40 report time.  I called in fatigued on Day 6 at approximately [3 hours prior]. I had just finished a 4 day trip from Day 2 to Day 5; with each day requiring subsequently later report times and the last 2 days requiring late or all night flying.  Additionally; even though this trip spanned 4 calendar days; it only included 2 sleep cycles.The previous 4 day trip created a significant cumulative sleep debt; as tracked my my Apple Watch. Day 1-Day 2; 6:38 of sleep at home prior to reporting for my 4 day trip. Day 2-Day 3; 8:44 of sleep in the ZZZ hotel. Day 3-Day 4; 5:26 of sleep after landing in ZZZ1; but I woke up at XA:00 am local time and had a XR:59 departure that night.  I attempted to take a nap prior to reporting for duty but I couldn't fall asleep and was awake for about 18 hours prior to flying a redeye. Day 3-Day 4; No sleep due to redeye.  I flew a redeye.  Extremely tired during the flight and took a 25 minute nap during cruise.  When I returned home; I slept from approximately XA:45 until the crew desk woke me from deep sleep shortly after XG:00.  After the crew desk woke me; I packed my bags and prepared for the new trip.  I attempted to go back to bed at XK:00 but could not fall asleep.  I realized that it was not safe for me to attempt to fly my 3rd transcontinental trip in 2 days so I called in fatigued. Day 5-Day 6; No sleep due to crew desk interrupting my recovery sleep from the previous redeye.Of note; my previous trip involved a significant cumulative clockwise circadian rhythm disruption in which every day required a later report time and each day flew later into the night.  Clockwise disruptions such as this are generally easier to tolerate compared to counter-clockwise disruptions.  However; this 4 day trip also was engineered to accumulate sleep debt.  Day 1 was 11 hours of duty and 20.5 hours of rest; which equates to a 31.5 hour long duty/sleep cycle.  Day 2 of my 4 day was 7:15 of duty and nearly 25 hours of rest; which was a 32 hour duty/sleep cycle.  Combined with each day's later duty periods; it was generally easy to fall asleep at the beginning of the rest periods.  However; it was not realistic nor possible to achieve 2 circadian rests or resets during the layovers; which meant that each day I would be awake for a longer period of time prior to reporting for late night flights.  This culminated when I reported for my redeye after being awake for 18 hours.  Upon landing I was absolutely exhausted and needed to catch up on the previous 4 days of missed sleep.  Unfortunately the crew desk interrupted this much-needed sleep and assigned me the new trip on Day 6.  In other words; the crew desk expected me to shift from my previous XM:00 pm and XS:00 pm local report times to a XA:40 am report time after disrupting my rest and being awake for over 12 hours.  Furthermore; the crew desk did not consider the larger picture of the 4 day trip and subsequent 2 day trip as a single 6 day long physiological marathon.  Considering the operational design of my previous 4 day trip; combined with a back-to-back 2 day trip; I do not think it's physiological possible or safe for anyone to endure such a disruptive 6 day period especially after the crew desk wakes you up in the middle of a much needed restorative sleep period.




Synopsis: Air Carrier First Officer reported they called in fatigued due to not being able to get sufficient rest from the previous four days flying schedule.