dairy = casein
casein takes longer to break down..
when u sleep your body is rebuilding itself.. its in major need of amino acids throughout the night..
if you were to, for example, eat chicken at 10pm and go to sleep, by 3am your body starts starving for nutrients and has nothing to restore muscle fiber with..
now in the case of casein, it takes 7-8hrs for it to break down and go through your system.. your body is fed longer when u sleep
the best thing to eat before bed, or near the latest meal of ur day is beef.. a good lean beef takes 18hrs to run through your system... so your body is getting a steady stream of amino acids as you sleep and therefore can repair and rebuild muscle...
hope that explains it