Your neurons are always active which is why your brain needs so much energy.
A child's brain uses more energy than an adult's. There are two reasons for this:
Your brain needs lots of energy because your brain cells are active all the time - keeping you alive, thinking and moving.
An adult brain requires approximately 30 Watts of power all day everyday in order to function - whether you are sleeping or sitting an important exam!
As an adult, about 20% of the energy from your food goes to your brain. Your brain needs energy to send signals along neurons (brain cells), send chemical signals from one neuron to another and to reset each cell after it has fired the message.