In a small bowl, dissolve the sugar in the warm water. Sprinkle the yeast over the water and let it sit for about 10 minutes, or until it becomes frothy.
In a large mixing bowl, combine the flour and salt. Make a well in the center and add the yogurt, vegetable oil, and the frothy yeast mixture. Mix until the dough starts to come together.
Turn the dough out onto a lightly floured surface and knead for about 10 minutes, or until the dough is smooth and elastic. If the dough is too sticky, add a little more flour, a tablespoon at a time.
Place the dough in a lightly oiled bowl, cover with a damp cloth or plastic wrap, and let it rise in a warm place for about 1-2 hours, or until it has doubled in size.
Preheat your oven to 475째F (245째C). If you have a baking stone, place it in the oven to heat up.
Punch down the risen dough and divide it into 8 equal pieces. Roll each piece into a ball. On a lightly floured surface, roll each ball out into an oval or round shape, about 1/4 inch thick.
If using, brush each naan with a beaten egg and sprinkle with sesame seeds or nigella seeds.
Place the naan directly on the hot baking stone or on a baking sheet. Bake for about 5-7 minutes, or until the bread is puffed and golden brown. If using a baking sheet, you may need to bake in batches.
Remove the naan from the oven and cover with a clean cloth to keep warm. Serve hot with your favorite Afghan dishes or enjoy it on its own.
Enjoy your meal!