Use a large mixing bowl as the batter will rise to double its original volume as it sits overnight. Put the warm water in the bowl and sprinkle in the yeast, letting it sit for 5 minutes.
Add the warm milk, melted butter, salt, sugar and flour to the yeast mixture and beat until smooth. I just used a good whisk but you could use a hand blender if you wanted. Just make sure there are no lumps.
Cover the bowl with plastic wrap and let it sit overnight at room temperature.
Preheat your waffle iron to high. (The standard kind, not Belgian.)
In the morning, beat in the eggs, then add the baking soda and stir until well blended. The batter is very thin at this point.
When your waffle iron is ready, pour in 1/2 - 1 cup batter, depending on the size of your iron. (Check the directions.)
Bake until done. They'll be crisp on the outside and still soft on the inside.
The batter will keep for several days in the refrigerator if you don't want to make them all the first day.