Casein
Casein is the main family of phosphoproteins in mammalian milk and the protein fraction that forms a curd when milk is acidified or treated with rennet. It occurs naturally in dairy foods and is sold as a protein powder. Casein is a complete milk protein, not a special overnight-only nutrient.
In milk, caseins are organized in structures called micelles. In the stomach, casein tends to form a clot that slows gastric emptying and amino-acid appearance compared with whey. "Slow protein" describes this average digestion pattern; it does not mean amino acids are released at one constant rate or that slower is always better.
Common products include micellar casein and caseinates such as calcium or sodium caseinate. They are processed differently and do not have identical texture or behavior. Casein powders are usually thicker than whey and are used in shakes, puddings, and blended protein powders. Nutrition facts, ingredients, and serving size remain more informative than a front-label promise of sustained release.
Casein before sleep has been studied because it can supply amino acids through the overnight fast. A systematic review of pre-sleep protein research found improved overnight protein-synthetic responses in specific study settings and possible training benefits, but the long-term evidence is limited and often compares added protein with no protein rather than with the same protein earlier in the day. The result may therefore reflect more total daily protein, not a unique bedtime property.
A normal protein-rich evening meal can serve the same practical purpose. Casein is useful when its texture, slower digestion, or timing fits preference and appetite; it is not required to "prevent muscle breakdown" while sleeping. Overall intake, resistance training, and a workable nutrient-timing pattern matter more than choosing one milk fraction by the clock.
Casein contains milk proteins and is unsuitable for milk-protein allergy. Pure casein powder may contain little lactose, yet composition varies and a low-lactose claim is not the same as allergy safety. People who experience gastrointestinal symptoms should check the full formula, portion, sweeteners, and thickeners instead of assuming casein itself is the only cause.