Back in the day my then boss once was bitten by the Wheel of Energy bug. Simply put, your total energy can be described as a circle which is divided into three segments: Work, leisure/hobbies and rest. For optimal energy the three should be balanced. If one segment is bigger than the other two, the weight of it will stop the wheel from spinning.
Dehydration is another issue. A hot environment, thick/dark clothes (sunshine on a breezy day in spring), efficient ventilation (aeroplanes) all can dry yourself without you noticing. Caffeine is a diuretic so coffee or coke can make things worse. Some common vegetables like watermelons and tomatoes also remove fluids from your system - handy if you have swollen ankles but not so much if you're already dry as a raisin. Excessive drinking of plain water (a gallon a day) can also be an issue as it can change your salt balance. Even more so if you sweat a lot.
