Casein Fuel your muscles whilst you sleep! The USN Casein provides you with a premium protein supplement, which releases into your body over an 8 hour period. Your body's highest state of recovery is during your sleep, the USN Casein is formulated to make the most out of this fact and give your muscles a slow release of protein throughout your sleep. Casein Fuel your muscles whilst you sleep! The USN Casein provides you with a premium protein supplement, which releases into your body over an 8 hour period. Your body's highest state of recovery is during your sleep, the USN Casein is formulated to make the most out of this fact and give your muscles a slow release of protein throughout your sleep.