Imagine, if you will, that Energy is your body's currency. Unlike ordinary money, you can't save up for a special occasion or a rainy day; instead, you receive a fixed daily allowance. You do, however, have an overdraft, the interest for which is extortionate, and the terms require you to repay the debt over the next few days+. This means that if you have used your Energy overdraft, you will often have significantly less Energy in your daily allowance for one or two days after.
You must use Energy to pay for everything you do throughout the day; this includes trivial tasks such as getting out of bed, cleaning your teeth, and preparing your meals. Eating well provides you with a small return for your Energy investment. spent preparing the meal. Naps are a slightly higher risk investment, since you must also invest Time, and your Energy return is not necessarily larger than or equal to your Time and Energy investment. Occasionally, the Energy bank shortchanges you and gives you less Energy in your daily allowance than you would normally have. Those days are usually bad days.
Every so often, you receive a bonus in your Energy allowance which you can use to complete extra tasks during the day, which would normally be too expensive. As with your normal daily allowance, if you don't use it, you lose it. There is some evidence to suggest that the healthier/more physically fit you are, the higher your daily allowance is; however, this does also require a bit of a gamble, as exercise can be very expensive. Especially if, like me, you have never been particularly fit.
The majority of customers banking with the Energy Bank receive quite a sizeable allowance, and rarely need to make use of their overdraft. More often than not, it is only used for things such as jetlag and special events such as weddings, birthdays or New Year's parties. There are, however, a significant number of people who, for one reason or another, receive a significantly smaller allowance. Most will suffer from long term illnesses such as MS or ME; people with depression, or on long term medication; and even insomniacs and shift workers.