http://www.sciencealert.com/watch-you-are-actually-two-brains-living-in-one-person

Everyone should learn about the nature of our minds and brains. While awake, your brain is by far the most expensive organ in the body, metabolically — it dominates the body with demands for energy and glucose, and the hemispheres compete for this fuel. The left hemisphere possesses a transport with which it can literally shut down the influence of the right, except for brief interruptions of its dominance and near tyranny. Some 70% or more of our metabolic energy funds the brain while awake. And when we sleep, the dominance inverts, and the gentle aspect awakens to dreaming, the metabolic costs fall precipitously, and the body can rest, heal and remake itself. So, too, the mind.