Studies indicate that, compared to casein, whey causes a greater increase in muscle protein synthesis when measured over a few hours. However, when measured over longer periods, the effect is not so clear cut.
For example, in one of the first comparative studies, protein balance over a period of 7 hours was significantly higher with casein compared to whey. One then may think that casein might provide a more anabolic (muscle-sparing) effect at night.
Researchers from the Netherlands had recreationally-active men and women perform a multi-set bout of resistance exercise at 8 pm. After exercise, they consumed 20 grams of whey with 60 grams of carbs. At 11:30 pm, they took another supplement containing either 40 grams of casein or a placebo. Blood measures were made while they were asleep, and measures of muscle protein synthesis were made the next morning. Plasma levels of essential amino acids were elevated throughout the night after casein ingestion but not placebo. Whole body protein synthesis over the night was positive after casein ingestion and negative after placebo. Compared to placebo, muscle protein synthesis during the night was 22% higher when casein was consumed before going to bed.
These results show that consuming casein before bed results in rapid and sustained elevations in blood levels of amino acids, and greater whole body protein balance and muscle protein synthesis.