A DH8-100 FO CLBS ABOVE HIS ASSIGNED ALT DEP PHL; PA.

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I WAS THE PF OF A SCHEDULED COMMUTER FLT FROM PHL TO LGA. UPON CHKING IN WITH DEP; WE WERE ASSIGNED TO CLB AND MAINTAIN AN ALT OF 7000 FT. AS WE WERE CLBING THROUGH 7900 FT; I REALIZED THE ERROR. I IMMEDIATELY DSNDED BACK TO 7000 FT. THERE WERE NO KNOWN TFC CONFLICTS AND WE WERE NEVER QUESTIONED BY ATC. I BELIEVE MULTIPLE PROBS CONTRIBUTED TO THIS OVERSIGHT: 1) THE CAPT WAS OFF FREQ COMPLETING COMPANY REQUIRED COMS. 2) I WAS ON DAY 3 OF WORK WITH THE FOLLOWING DUTY HOURS: DAY 1 - 13 HRS. DAY 2 - 13 HRS. DAY 3 WAS 8.5 HRS AT THE TIME OF THE OCCURRENCE. 3) THE AUTOPLT WAS DEFERRED RESULTING IN HAVING TO HAND FLY THE ACFT ON THIS LEG; AND THE PRECEDING 4 LEGS. 4) THE YAW DAMPERS WERE DEFERRED RESULTING IN EXTENSIVE ACFT MOVEMENT IN THE TURBULENT CONDITIONS. 5) PRIOR TO DEP FROM PHL; WE HAD TROUBLE GETTING A GPU FROM THE RAMP PERSONNEL. THIS WAS A NECESSITY DUE TO A DEFERRED APU AND RESULTED IN A LATE DEP. 6) AFTER SHUTTING THE DOORS AND COMPLETING THE PAPERWORK; ADDITIONAL BAGS WERE LOADED RESULTING IN HAVING TO AMEND THE PAPERWORK. BECAUSE OF THESE CONTRIBUTING FACTORS; I BELIEVE MY INATTENTIVENESS WAS A RESULT.