Whey protein ingestion results in a higher peak concentration of blood amino acids compared to casein protein, whereas casein leads to more prolonged elevations in blood amino acids; however, the total amino acid availability, measured as the area under the curve, is similar between whey and casein, and there are no significant differences in myofibrillar or mitochondrial protein synthesis rates during overnight recovery following protein intake.

Study: Pre-sleep Protein Ingestion Increases Mitochondrial Protein Synthesis Rates During Overnight Recovery from Endurance Exercise: A Randomized Controlled Trial

The study looked at whether whey or casein protein before bed helps muscles recover after exercise, and found both work equally well, which supports the idea that they’re similar overall even if they act differently in the body.

Study: Myofibrillar and Mitochondrial Protein Synthesis Rates Do Not Differ in Young Men Following the Ingestion of Carbohydrate with Milk Protein, Whey, or Micellar Casein after Concurrent Resistance- and Endurance-Type Exercise

The study shows that whey gives a quick spike in amino acids and casein gives a slower, longer release, but both lead to the same overall muscle and mitochondrial building over time.

Study: Pre-sleep Protein Ingestion Increases Mitochondrial Protein Synthesis Rates During Overnight Recovery from Endurance Exercise: A Randomized Controlled Trial

The study looked at whether whey or casein protein before bed helps muscles recover after exercise, and found both work equally well—just like the claim says.