The time you spend sleeping is one of the longest periods your body goes with nutrition. It’s been theorized that drinking a slowly digested protein like casein before bed can help fuel recovery from resistance training. A review of studies published in the journal Frontiers in Nutrition looks at research on this subject.
In a 2015 study, 44 healthy young men consumed a shake containing 30 grams of casein and 15 grams of carbohydrates before bed every night during a 12-week weight training program. Some were given a calorie-free placebo. While both groups increased squat one rep max and built up quad muscle size, subjects in the protein before bed group gained significantly more muscle strength and size.
A smaller study of 26 experienced weight lifters compared casein before bed to casein in the morning. After 8 weeks, subjects who consumed casein before bed gained 2.6 pounds of muscle compared to the 0.9 pounds gained by subjects in the morning protein group. This difference was not considered statistically significant.