B737-700 CAPTAIN REPORTS FLIGHT ASSIGNMENT UNSAFE DUE TO UNREALISTIC SCHEDULED FLT TIME ON FIRST LEG COMBINED WITH FATIGUE DUE TO ALTERNATING LATE NIGHT AND EARLY MORNING STARTS TO DUTY DAYS.

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TRIP PAIRING 1234 ZZZ1-ZZZ2 IS AN UNSAFE PAIRING. THE PROB I HAVE WITH THIS PAIRING IS THAT IT APPEARS THAT THIS PAIRING DOESN'T REALISTICALLY REFLECT AN ACCURATE BLOCK TIME ON FLT ZZZ1-ZZZ2. AFTER COMPLETING THE FLT; OUR ACTUAL BLOCK TIME WAS 6 HRS 40 MINS. TO COMPOUND PROBS; RATHER THAN ENDING THE DAY'S WORK IN ZZZ2; IT CONTINUES ON TO ZZZ3. I DIDN'T REALIZE HOW FATIGUED I WAS UNTIL I WAS ON APCH INTO ZZZ3. HAD I KNOWN IN ADVANCED I WOULD HAVE ARRIVED ZZZ3 AS FATIGUED AS I DID; I WOULD HAVE NEVER DEPARTED FROM ZZZ2 DUE TO CREW FATIGUE. THE PAIRING NEEDS TO REFLECT A MORE ACCURATE BLOCK TIME ON FLT ZZZ1-ZZZ2. ALSO; AFTER ARRIVING ZZZ2; THE CREW SHOULD GO INTO CREW REST RATHER THAN CONTINUE ON TO ZZZ3. IT IS VERY HARD ON BODY RHYTHMS AND CLOCKS FLYING AN ALL-NIGHTER ON THE FIRST DAY OF DUTY; ONLY TO BE FORCED TO SWITCH MY BODY'S CLOCK TO AN EARLY MORNING DEP THE FOLLOWING MORNING. WHEN BUILDING PAIRINGS; THE COMPANY NEEDS TO TAKE INTO ACCOUNT BODY RHYTHMS AND SLEEP PATTERNS. FATIGUE DUE TO INAPPROPRIATELY BUILT PAIRING.