ACCORDING TO AYATULLAH SISTANI’S RULING, BASED ON PRECAUTIONARY MEASURE, YOU SHOULD START YOUR FAST (STOP EATING) AT IMSAK TIME WHICH IS AT LEAST 10 MIN PRIOR TO THE FAJR TIME AND PERFORM FAJR PRAYERS 10 MIN AFTER FAJR TIME. YOU CAN USE THIS PRECAUTIONARY TIME BETWEEN IMSAK AND FAJR SALAT TO OFFER YOUR TAHAJJUD PRAYERS.)