How do you improve your diet?
I've noticed a similar effect. E.g. if I don't consume enough carbs, I'll start feeling more lethargic. But then again, the most optimal I've felt, or 'least non-optimal', is not something I'm happy with.
A balance of protein, fats, carbs, an unbalance of them, consuming fish oil, not consuming sugars, consuming veggies, etc, all the general principles seem to keep me from not dipping into lethargy, but nothing more.
My energy at the gym is good on most days, its my general energy levels I'm not satisfied with.