Throughout my working life, there've been times when I've found myself becoming sleepy, not enough to actually fall asleep, but enough for my eyes to feel heavy, my thinking to become sluggish and my motivation to become non-existent. Even way back when I was order picking, a physical labour job requiring me to walk miles through a giant warehouse every day, I'd find myself becoming sleepy while on my rounds, even while walking.
Worse, only certain sudden stimuli could jerk me back into near-full alertness - like the phone going, my manager addressing me, or other random things. Other than that, it was near total shutdown and I'd frustrate myself trying to wake myself up, without success.
When I came across the Paleo diet, and low-carb dieting in general, I studied the effects of different food types on blood sugar, and the energy slump that's suffered after a meal heavy in processed carbs. Sandwiches, being the first choice for many office workers, are a prime example. Dr Briffa posted about them recently in his blog.
Since making my dinners low-carb, I haven't had problems with energy slumps in the afternoon, and having a fried breakfast as opposed to cereal has had the same effect on my mornings. Now my energy remains constant throughout the day and I'm finding myself able to concentrate more.
My dinners now include smoked fish (although you risk the wrath of your fellow workers as it does smell strongly), bacon fried that morning, cold meats, cheese, leftover roasts and low-carb salad greens.
As for breakfast, I usually have bacon and mushrooms fried in butter. I tried to like eggs, but I can't be doing with either the taste or the smell. I finish with a slab of full-fat cheese. If I'm in a rush, I make sure I at least have the cheese on my way out.
If you're finding that there's times of the day when you doze off, you might consider switching high-carb elements of your daily meals to low-carb alternatives.