B757 FLC LOST VISUAL CONTACT WITH THE COMMUTER ACFT AHEAD ON PARALLEL RWY VISUAL APCH DUE TO FLYING THROUGH A CLOUD AREA WITHOUT NOTIFYING TWR.

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DURING APCH TO SFO; BAY APCH ASKED US TO MAINTAIN VISUAL SEPARATION FOR COMMUTER ACFT LNDG SFO RWY 28L. APCH ASKED US IF WE COULD SEE THE ARPT OR SAN MATEO BRIDGE. WE INDICATED 'BRIDGE IN SIGHT.' APCH CLRED US VISUAL APCH RWY 28R; MAINTAIN VISUAL WITH ACFT RWY 28L; WHICH WE ACKNOWLEDGED IN SIGHT. OVER THE SAN MATEO BRIDGE AND JUST OUTSIDE; IT WAS A LOCALIZED AREA OF CLOUD COVER. AT ABOUT 2500 FT; WE ENTERED THE CLOUD DECK AND LOST SIGHT OF PARALLEL TFC WHICH WAS APPROX 2 MI AHEAD OF US. BEING THE PF; I STAYED WELL R OF THE LOC AND WE MAINTAINED SEPARATION USING TCASII. WE WERE OUT OF VISUAL CONTACT WITH THE OTHER ACFT FOR MAYBE 45 SECONDS OR SO. HOWEVER; AT THE BRIDGE; 1500 FT AGL; WE BROKE OUT AND REACQUIRED THE OTHER ACFT. I THEN INTERCEPTED THE LOC AND MADE A NORMAL LNDG. IN RETROSPECT; WE SHOULD HAVE NOTIFIED ATC IMMEDIATELY OF LOSS OF VISUAL CONTACT. HOWEVER; THIS WAS THE END OF THE DAY AND GET HOMEITIS WAS TAKING OVER. THIS SCHEDULE STARTED WITH AN ALL NIGHTER THE NIGHT BEFORE FROM SFO-ORD AT XA00 AND THOUGH THERE IS ADEQUATE REST IN THE SCHEDULE; I USUALLY ONLY SLEEP FOR 5-6 HRS. (I'VE FOUND MOST PLTS AGREE THIS IS ABOUT ALL THE REST THEY CAN GET ON ONE OF THESE SCHEDULES.) THIS SCHEDULE WAS PRECEDED BY ANOTHER ALL NIGHTER WHICH MEANT I HAD 2 SHORT SLEEP CYCLES IN A ROW. SO FATIGUE WAS A MITIGATING FACTOR; AS WELL AS THE DESIRE TO GET HOME TO MY OWN BED!