CAPT OF A B737 INADVERTENTLY RESET HIS ALTIMETER TO THE LCL SETTING WHEN ABOVE FL180 CAUSING THE ARTCC CTLR TO BELIEVE THAT HIS ACFT HAD DSNDED BELOW ASSIGNED ALT. HOWEVER; THE FO; FLYING; NOTICED CAPT'S MISTAKE AND FLEW THE ACFT AT THE CORRECT ALT BY HIS ALTIMETER. THE CAPT IMMEDIATELY CORRECTED HIS ALTIMETER SETTING REFLECTING THE CORRECT ALT TO ATC.

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CLOSE TO THE END OF A LONG DAY AND THE THIRD DAY OF A 4 DAY TRIP; WE WERE DSNDING INTO PHILADELPHIA; PA. THE FO WAS FLYING. WE WERE INSTRUCTED TO CROSS JOHNSTOWN VOR AT FL190. THE FO STARTED DOWN IN PLENTY OF TIME TO MAKE THE XING. I WAS PREOCCUPIED WITH SOME PERSONAL FAMILY ISSUES. WHEN I HEARD THE CTLR TELL ANOTHER ACFT THAT THE ALTIMETER WAS 30.74 INCHES; I; FOR SOME UNKNOWN REASON; STARTED TO RESET THE CAPT'S ALTIMETER TO 30.74 INCHES. THE FO NOTICED WHAT I WAS DOING AND DISENGAGED THE AUTOPLT. HE THEN LEVELED OFF AT FL190 USING HIS ALTIMETER FOR REF. THE CTLR ASKED OUR ALT AND I RESPONDED 'FL180 -- NO; FL190!' THE CTLR CLRED US TO FL180. I RESET MY ALTIMETER TO 29.92 INCHES AND WE DSNDED TO FL180. I HAVE NEVER DONE THAT BEFORE IN 30 YRS OF AIRLINE FLYING. THANKFULLY THE FO WAS SHARP ENOUGH TO KEEP US FROM BUSTING OUR CLRNC; ALTHOUGH IT MAY HAVE APPEARED SO TO THE CTLR AS THE ALT RPTING COMES OFF OF THE CAPT'S ALTIMETER. THE ONLY CORRECTIVE ACTION I CAN RECOMMEND IS TO INCREASE VIGILANCE ON LONG DUTY PERIODS.