Most people that discontinue dairy for diet reasons do not include their protein shake in that group.
You should read this article Introduction
Casein is the predominant protein found in milk. While whey makes up about 20% of the protein content in cow milk, casein makes up the other 80% . Casein protein has been used by athletes for decades for its anticatabolic (muscle sparing) properties.
Casein is usually found in three forms, calcium caseinate, micellar casein, and milk protein isolate. Calcium caseinate is considered to be lower quality, while micellar casein and the casein in milk protein isolate are identical. This makes milk protein isolate more economical given a choice between the two. In comparison to whey, casein is not as high quality by some conventional standards of measuring protein quality. The biological value of casein protein is about 70 compared to 100 for whey. However, milk protein has a biological value of over 90. And if we use the PDCAAS, a more modern method of determining protein quality, all three of these proteins (casein, whey, and milk) are on equal footing at 1.00.
What is unique about casein protein is the slow digestion rate. Studies that compare whey and casein administered in the workout period inevitably find that whey protein exerts a far stronger anabolic response in this case . However, when casein is administered at other periods the strength gains are twice of those seen with whey . This is because while whey stimulates protein synthesis much more than casein in the short term, casein causes a sustained release of amino acids, preventing muscle breakdown for a more extended period of time . This makes whey the protein of choice for the workout period, with casein preferable at other times, especially before periods of fasting (such as sleep). Because casein does not have a pleasant taste, a blend of casein and whey is generally used. An additional reason not to completely ditch the whey is that it has a plethora of health benefits that casein does not have, although some peptides in casein do have free radical scavenging activity .
There are many "sustained release protein" products on the market. However, many of these probably do not contain as much casein as they appear. Companies want to avoid putting too much casein in a product for a few reasons. First, casein is more expensive than whey. Second, casein has a distinct, malty taste which many find unpleasant, and the taste is not easy to mask. However, because a protein with high casein content is sought after, the advertisement may make it appear as though the protein blend contains more casein than it actually does. To tell if a product is high in casein, milk protein isolate or some form of casein should be the first ingredient. Even then, low casein content may be masked by use of many different protein sources. For example, if the ingredients section of a protein lists, in order, milk protein isolate, whey protein concentrate, whey protein isolate, egg protein, and soy protein, the casein content may be lower than 20%.
Note this line:"before bed, you want the most sustained release of amino acids possible."