When we are talking of energy in a wide range of levels, that goes from the food intake to the molecular level, involving protons and ions cascades. The energetic input for the body starts when food is ingested, through its process in the digestive system the food is decomposed into smaller compounds, that will be transported into the different organs and cells of the body. The primary source of energy in humans is glucose (sugar), this molecule follows a metabolic process where ATP and NADHP, are generated, which are none other than the main energetic molecules in the cell. The order of processes is as follows: glucose enters glycolysis, then goes to Krebs cycle, and finally to mitochondrial electron transport chain, generating a maximum of 38 ATP molecules per glucose molecule.
Example
When we are sleep deprived, the bain needs a higher energetic intake to function properly, therefore, we increase our food consumption, generally, we seek, sugary food. This is because glucose enters immediately to the metabolic route described before, and energy is sented to the brain.