It's nutrition and sleep. See my "
smoothie" thread in the Commuting Forum. A "smoothie" can sound like a stupid thing (maybe it's the word-
smoothie?) but it's a quick dependable way to get those basic nutritional needs on a daily basis. I do use a protein supplement- not too much but a touch.
Check your carb intake. Sometimes the riding makes you crave carbs but if the balance of carbs to protein is too high on the carb side it can give you an energy lag.
Then you really do need to sleep 8 hours a night as many nights of the week as possible. And hydrate, hydrate, hydrate.
One other thing. The regular commute, as good as it is for you, can drag you down unless you change it up a bit. Go for a fun ride once in a while. Then do a kick ass 35 miler every so often where you burn at a high speed, high reps ride. get proper rest and food after those rides and when you get back to your commute at your nice pace you may feel a new found energy.