There are a lot of igood deas up there, but here's my take. If you are going to do just egg whites, you still need something else in there. Salsa and broc is a good option (or eliminate the sodium from the salse if cutting as suggested). Straight Tuna is not a good idea even if you are overweight, you don't want your body to go into ketosis as that is indicative of a negative nitrogen balance where no muscle can be built and defeats the whole purpose of eating protein before bed. It is high in mecury (that's why it is siggeested to limit your intake to 2 times a week at most) and high in sodium (although you can get low sodium and run in through a strainer). Your body will do this if all you eat is protein and you will not build muscle.
I do the same exact thing you do (casein normally) and I switch it up with eggs every once in a while as a end of day treat. I usually use 2 whole eggs and then 1 cup of eggs whites giving me around 25-30 grams of quality protien and the necessary fat to make sure my body has other things to burn instead of the protein. It is an increased metobolic process to burn protein over fat, so erroring on the side of caution you could cut it down to one egg, but if you run out of fuel, you will start to metabolize the protein and give up the positive nitrogen balance you need to create muscle.
A good gauge for this is how hungry you wake up. I know if I wake up at 3am to eat (which I will do if I am hungry), then I prob skimped on the scoop of casien or if it was eggs, I didn't put enough in there. It is important to listen to your body, it will tell all and the little bit of fat you will get from the egg yolk will not add any fat to you, it will just make sure you are using the protien for building muscle as intended.
On a final note, make sure you are mixing you casein with some sort of fuel. I love pineapple juice because it tastes great and is natural fructose. I'm very active so my body needs the liver glycogen replinished often and this works great. Plus, your body will use that glycogen before it will let you enter ketosis, so it is a good bet to get the most use out of your expensive casien.
Just my two cents
