Studies suggest that ingesting protein just before overnight sleep improves muscle gains in response to resistance training. However, does the timing of the protein intake really matter that much? Seems so, according to a new review of recent studies which found that overnight sleep is a unique nutritional window for boosting muscle gains.

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The review was led by Dr. Tim Snijders, Assistant Professor at Maastricht University. In 2015, Snijders and colleagues performed their own investigation of muscle gain from nightly protein intake. Their study involved 44 healthy young men on a 12-week lifting program, half of whom were given a pre-sleep protein shake consisting of 30g of casein and 15 grams of carbs, while the other half received an energy-free drink. Both groups grew bigger quads and could lift more but the protein-before-bed group saw better gains in both muscle strength and size.

Snijders’ study begged the question: is the timing of the protein shake before bed important or is it all just about the higher intake of protein and calories? That is difficult to show directly because “a huge number of participants would be needed to prove whether a difference might exist in response to pre-sleep protein, versus protein intake at other times of the day,” explained Snijders.

However, this most recent review of relevant scientific literature suggests that there are numerous indirect indicators that pre-sleep protein is specifically important for muscle gain, with sleep playing a unique window of opportunity.

When muscles suffer trauma from resistance training, this disruption activates satellite cells located on the outside of muscle fibers to proliferate at the injury site. These cells perform the biological function of repairing or replacing damaged muscle fibers, often leading to an increase in muscle fiber cross-sectional area (hypertrophy). In order to sustain hypertrophy, muscle cells need amino acids from protein present in the blood. However, the body does not release amino acids at near-constant circulating levels. Rather, they fluctuate in peaks and valleys depending on the amount of ingested protein.

“A survey of over 500 athletes found they were typically consuming at total of more than 1.2g protein per kilo of bodyweight across three main meals, but only a paltry 7g of protein as an evening snack. As a result, lower levels of amino acids would be available for muscle growth during overnight sleep,” Snijders commented on the results of one of the studies included in his review.

Evidence suggests that pre-sleep protein intake allows muscles to absorb more amino acids at night — and this doesn’t mean that there will be less during the day.

“The muscle-building effects of protein supplementation at each meal seem to be additive. In one study we found that the consumption of ample amounts of protein (60g whey) before overnight sleep did not alter the muscle protein synthetic response to a high-protein breakfast the following morning,” Snijders said.

“What’s more, others have shown that adding a protein supplement at bedtime does not affect appetite the following morning – so it is unlikely to compromise total protein or calorie intake.”

Bedtime protein doesn’t seem to make you fat either. Surprisingly, it might have the opposite effect by speeding up metabolism. In one study, researchers compared an 8-week morning vs evening casein program and found no difference in fat mass between the two programs.

“Supporting this, another group found in 11 young active men that a pre-sleep casein shake actually increased the rate of fat burning the following day. This might be because casein ingestion reduces the insulin response to subsequent meals, which pushes your body to use more fat,” Snijders said.

The review also found that bedtime protein doesn’t interfere with sleep quality or drive onset latency.

“In conclusion, protein ingestion prior to sleep is an effective interventional strategy to increase muscle protein synthesis rates during overnight sleep and can be applied to support the skeletal muscle adaptive response to resistance-type exercise training,” the authors concluded.

The findings appeared in the journal Frontiers in Nutrition