COMMUTER ACFT DSNDS BELOW ASSIGNED ON APCH.

Narrative

I WAS THE PF ON FLT FROM ALB TO JFK. WE WERE DSNDING FROM 4000 FT TO 3000 FT. ATC STARTED TO GIVE US VECTORS IN FOR WHAT WE THOUGHT WAS SETTING UP OUR APCH INTO JFK. DURING THIS TIME WE WERE ALSO FINISHING OUR DSCNT CHKS. WHILE I WAS SETTING MY RADIOS FOR THE APCH; THE CAPT STARTED TO PULL ON THE YOKE AND STATED THAT WE WERE SUPPOSED TO STOP OUR DSCNT AT 3000 FT. WHEN I LOOKED UP I SAW THAT I HAD LET THE ACFT DSND TO 2700 FT. JUST AT THAT MOMENT; ATC GAVE US ANOTHER VECTOR TO 180 DEGS AND STATED THAT HE WAS GOING TO HAVE DEP TFC OUT OF REPUBLIC AND HE WANTED TO KEEP IT CLR OF US. THE CAPT CONFESSED OUR MISTAKE TO ATC BUT ATC DID NOT SEEM THAT CONCERNED. WHEN ATC NOTICED THAT WE WERE BACK AT 3000 FT; WE WERE GIVEN A TURN TO 230 DEGS. ATC AND MY CAPT DID NOT SEEM TOO CONCERNED SINCE NO CONFLICT EXISTED. BUT I WAS CONCERNED SINCE I HAVE BEEN FLYING IN THIS AREA FOR A NUMBER OF YRS. THE WORKLOAD WAS NOT THAT HIGH AND YET I PASSED THROUGH AN ASSIGNED ALT. AFTER WE GOT ON THE GND; I TRIED TO FIGURE OUT WHAT WENT WRONG. FIRST; THE SHORTS 360 ACFT THAT WE WERE FLYING HAD AN MEL'ED FORWARD FREON SYS. SECOND; THE TEMP AND HUMIDITY WERE VERY HIGH. THIRD; DUE TO CONVECTIVE ACTIVITY AND QUICK TURNS; THE CREW WAS UNABLE TO DRINK ANY FLUIDS. THEREFORE; WHAT HAD HAPPENED WAS THAT I WAS STARTING TO SUFFER FROM HEAT EXHAUSTION. BACK ON THE GND WE HAD SOME TIME TO REST. I STARTED TO DRINK SOME WATER AND NOTICED AN INCREASE IN ALERTNESS AND GENERALLY WAS FEELING BETTER. THE BOTTOM LINE IS THAT I LET MYSELF GET AFFECTED BY THE HEAT WHEN I SHOULD HAVE KNOWN THAT ON A HOT; HUMID DAY I SHOULD BE MORE AWARE OF THE AFFECT OF THE HEAT.