When I first took my current job with Red Hat, the office I was to work in was a commute of about 55 miles. I have an apartment that is very nice and affordable, but I had chosen it mainly for its proximity to job I had at the time. Since I couldn't find a good deal in a comparable apartment in San Francisco, I shrugged off the commute and dealt with it.
What I didn't notice was that the commute had gradually worn down my energy level through the time spent in the car, in transit.
Well, for almost exactly a year now, I've worked out of a different office, one only about 11 miles from the same apartment. Shorter commute. Less time in the car. I assumed the only real difference would be more free time. I was wrong.
The gradual drain on my energy level over the 18 months of the longer commute had led to me being more inclined to eat out, often times at more fast-food-ish places. KFC, Burger King, etc. It was a self-feeding cycle, since crap food would have as much or more effect on my energy level as the time spent in the car. But you often don't notice these gradual things. Since my commute shortened, I've been cooking for myself more, and eating out much, much less. I don't claim to be a terribly health-conscious cook, and sometimes dinner is little more than pasta with tomato sauce and a lot of parmesan. Other times, I go the whole salad, two servings of veggies and meat route.
Snacks have changed a lot, as well. I don't buy as much junk (the occassional pint of Ben & Jerry's), but rather I buy a lot of fresh fruit. I shop every few days instead of my student-days practice of shopping once per paycheck, for two weeks at a time.
By itself, this really isn't all that my body and health need. I should exercise a lot more than I do. And I could do with a better daily schedule, making better use of the time that I'm awake and making less effort to stay up late so often.
Don't underestimate the significance of what you're eating on your overall energy level and alertness.