Casein protein is the primary protein found in milk and makes up to 80% of the protein in the glass.
The other 20% is made up of whey protein, so milk itself is a great muscle builder.
But to use specifically for muscle building, there are protein powders available.
Casein protein is a great supplement to use instead of whey at bedtime since it has a slower digestion and absorption rate.
This provides a more steady supply of amino acids for the body to use for processes while you sleep, meaning that the body doesn’t take them from muscle to function.