I used Vitathion in the mornings as I was pretty much asleep on my desk.
I got it on a script, but you can buy it at most chemists. It is a sachet, dissolve in water and drink. It has an orange flavour, so not too bad tasting (if drank quickly).
But it is pretty good as an energy booster.
Also keep your energy up by eating or snacking on small things during the day to try and stop your sugar levels dropping i.e. bag olf peanuts, small chocolate bar, apple, banana or what ever.