Energy Level thoughts
If you buy the classic American/Euro diet advice to eat lots of grains, carbohydrates, low fat you'll need to eat fairly often or you will have a low blood sugar energy problem. Have some nuts, snacks available and eat, it can really help.
An alternative strategy is the very low carbohydrate diets (Atkins, etc). MOST people report increased energy after a couple of weeks of adaptation. There is no credible evidence that a well formulated low carbohydrate diet is harmful (and I'm an MD.) Some people adapt so well that they can fast all day and not lose energy. If you try this, get a good book and follow it; just slashing carbohydrates without a plan doesn't work so well (I've tried it.) I've been on an Atkins-like plan for a year and love it.
Then in addition to diet: Exercise often helps, at least exercise in moderation.
Taking a five minute outdoor break helps me as does a 5 minute prayer/meditation break. The Pomodoro plan folks build a five minute break into every half hour!
Kelly is absolutely correct that sometimes the drain is something on your mind that is sapping your energy; do a mindsweep or do the thing you are dreading. Or alternatively imagine something great and work on that--sometimes that breaks my tired cycle.
How much sleep do you get? People really need 7-8 hours; many of us (remember I'm a doc, we're all crazy) fool ourselves into thinking we can do it on 5 or 6 hours. I'm so much more productive (and less whiney) if I just go to sleep at 10.
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