Micellar casein, the type most often found in supplements, is casein in its natural form. The micelles are small spherical structures with an outer layer that prevents casein from dissolving in acid and water. The micellar structure is the reason that casein forms a gel in stomach acid, making it digest more slowly. To reap the benefits of a sustained, slow release of amino acids, micellar casein is the best option.
Casein hydrolysate (also called hydrolyzed casein or casein peptides), on the other hand, is processed so that the micelles are already broken down and is a form of predigested casein. Casein hydrolysate has a similar absorption time to whey protein and so is not useful as a presleep protein supplement.[1][2]