Which vitamins and adrenal supplements are recommended to take to improve concentration, sharpness and focus?

I feel quite lethargic at work and find it difficult to sharpen my mind and concentrate. I know small amounts of caffeine help the brain stay alert, but are there any supplements that would aid this? I have heard omega 3, CLA supplements (see more details here) and cod liver oil are good, but I’m not sure if this is true or what doses. I’m about to start a new job and I want to be on the ball, not daydreaming.