On the night before breakfast, combine the warm water, sugar and yeast in a large bowl. Let stand for 5-15 minutes, the yeast will proof and turn the mixture bubbly. If you are certain that your yeast is alive you can skip the wait time and move on to the next step.
Add the milk, melted butter, salt, and flour and blend until smooth. A beater or emersion blender makes quick work of this.
Cover the bowl with plastic wrap and let the mixture sit overnight at room temperature. The batter will triple in size and then collapse back down so make certain that the bowl is large enough to accommodate the reaction.
In the morning preheat the waffle iron then beat in the egg yolks and baking soda.
Whisk the egg whites until they hold soft peaks then fold them into the batter.
Oil the waffle iron and pour ½ to ¾ cup of batter (or the amount recommended for your waffle iron) into the iron. Cook until the waffle is golden brown.
Keep the waffles in a warm oven until time to serve. If you place them in a single layer on a rack they will stay crisp.
Notes
Any leftover waffles can be cooled then wrapped in plastic and frozen. They reheat and crisp up well in a toaster.