Mix together all purpose flour, whole wheat flour, salt, sugar and yeast. To this add the 1.5 cups of warm water. Stir well till incorporated using a wooden spoon or a spatula. Cover with a cling film and set aside for rising at a warm place undisturbed from 12-18 hours. It should be more than double in size. I typically leave it on the counter on a warm day and inside by oven with light on cold days.