OPDEV DSCNDED BELOW ASSIGNED ALT.

Narrative

WE HAD BEEN FLYING SINCE XA00 AM ON RETURN LEG TO SOUTH CAROLINA FROM MICHIGAN; ASSIGNED 15000 FT. CTR ASKED TO SAY ALT. WE NOTICED WE WERE ABOUT 1000 FT LOW; IMMEDIATELY (20 SECONDS) CLBED BACK TO PROPER ALT. NOTHING MORE WAS SAID BY ATC CONCERNING DEV. THERE WERE NO TFC CONFLICT. BOTH PLTS HAD BEEN UP (AWAKE) FOR ABOUT 24 HRS (COMBINED FLT TIME AT ABOUT 16000 HRS). WHAT HAPPENED IS; WE THINK: GOT AN ALTIMETER CHANGE DISCONNECTED ALT-HOLD TO CORRECT ALT. FORGOT TO REENGAGE ALT HOLD; THIS STARTED WHAT WE FEEL WAS A VERY SLOW (100 FPM) RATE OF DSCNT. WE CAN'T UNDERSTAND WHY WE DIDN'T NOTICE (EXCEPT FOR LACK OF SLEEP) AND WHY DID IT TAKE ATC SO LONG TO SAY SOMETHING. FLYING UNDER FAR 91 THERE ARE NO DUTY HRS RESTRS COVERED SINCE DON'T APPLY WHEN YOUR COMPANY WANTS MANUFACTURED GOODS DELIVERED TO CUSTOMERS; SO WHY CAN'T THE FAA MAKE UP DUTY HRS RULES THAT APPLIES TO FAR 91 ALSO; KEEPING IN MIND THAT CORPORATE PLT MIGHT HAVE OTHER DUTIES BESIDES FLYING. BOSSES DON'T LIKE TO HEAR THAT THE PLTS ARE TIRED; WHEN THERE IS NO RESTR ON DUTY HRS.