The body uses ATP (adinosine tri-phosphate) producing energy....with one of the phosphates being broken off which then produces a unit of energy, then another phosphate is broken off which produces another unit of energy - then through a conversion of glucose (a form of sugar) two new phosphates are formed and attached back to that molecule - making it have three phosphates again, ready to be used for energy again.
When a person has exhausted their glucose stores (from their muscles and blood stream), then the body starts converting fats and proteins into glucose to rebuild these ATPs.
So, after one has exercised strenuously enough to deplete all these glucose stores, the body finally switches into fat burning. Important for people to know who are trying to lose weight.
There is a lag-time in converting the fats to glucose first, so it's felt as a tiredness. Also, when the glucose stores are that low, hunger hormones (grehlins) are released, so people get really hungry. That's why it's hard for many people to lose weight - they experience tiredness, brain fog, and hunger first - great discomforts. There are some tricks to help get past this, though.
Anyway, for the rest of us who want to keep our energy levels flowing, like children who don't want to lose weight, and for athletes who want to have fuel for their entire workout, replenishing with foods that turn quickly into glucose is what to use.
When people drink regular energy drinks, or soda pop, etc., they are drinking in large doses of sugars/glucoses all at once - since they don't have fiber in them, like bananas and other fruits have, to meter and regulate the sugar releases. With this rapid intake of sugars, the body quickly releases a lot of insulin hormone to handle the sugars - and this quick release of a lot of insulin is a key trigger for making the body go into fat-storage mode. Then, as sugars are pulled out of the blood stream and into storage, a tiredness actually results.
Many people then keep pouring down more sugary drinks, to keep their energy level up, which is mainly just giving the body more sugars to convert to fats for storage.
The healthy way to keep a steady stream of glucose in proper levels going in to the body to continuously fuel it, is through whole fruits. Bananas break down quite rapidly, more rapidly than other fruits, while still containing fiber so insulin doesn't trigger fat-storage mode. This makes bananas an ideal fruit to use for a natural, healthy energy drink.