All living beings require energy to perform their daily functions, including sleeping. Over time, species have developed different mechanisms to increase their energy efficiency. The tendency in organisms to conserve their energy is what ensures they will have extra to draw on in times of increased need. For humans, we have to be careful though, to

In Homo sapiens, the brain accounts for about 2–3 per cent of total body weight, but it consumes 25 per cent of the body’s energy when the body is at rest. By comparison, the brains of other apes require only 8 per cent of rest-time energy. Archaic humans paid for their large brains in two ways. Firstly, they spent more time in search of food. Seco

the brain seeks always to maximize its use of energy. Clearly, activities that can be performed easily require less investment of energy than those that demand greater time, effort, and thought.