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exercise.
    Unnecessary-for-immediate-survival brain operations like having coherent thoughts are sacrificed in order for the brain to be able to maintain vital functions like respiration during a drop in glucose, electrolytes, or water. Confusion and hallucinations are also warnings from our brain that we are dangerously close to doing damage to our bodies. The next step is passing out. This is the brain’s last-ditch way of protecting our bodies from exercising to death.
    It doesn’t always work. Every year several participants in marathons die because they inadvertently pushed their brains and bodies beyond certain critical limits. The brain will keep trying to consume its disproportionate share of your body’s energy. That’s why when your body runs out of energy you become a drooling zombie.
    Now imagine that running yourself to death in a marathon is a compressed version of your entire life.
    During the marathon, as you approach the limits of your body’s capacity to withstand stress, your brain will keep giving you warnings. Your muscles will feel fatigued, and you will start to have an overwhelming urge to stop. You may become disoriented and have momentary lapses in awareness.
    Some people can override these warnings and push themselves past the point of no return. Over the long term in a less intense, but no less insidious way, our brains are constantly warning us that we work far too much. On the time-scale of a lifetime, constant stress from overwork raises your risk of depression, heart-disease, stroke, and certain kinds of cancer. It’s a long, horrible list.
    Yet we feel obliged to risk our long-term health in order to work extremely hard at jobs we don’t particularly enjoy in order to buy things we don’t particularly want. This is otherwise known as free-market capitalism. According to politicians, CEOs, and bankers, this is also supposedly the highest form of social organization that human beings have attained.
    Few people fear being overweight as much as they fear terrorism, even though statistically being obese is much more of a threat to your life than terrorism. We do not know how much stress and overwork contribute to shortened life-spans. But we do know that obesity and sitting all day at your desk with a low level of constant stress are related. If you knew being idle (preferably while lying down on a blanket under a tree with a nice bottle of wine) for more hours of the day could add years to your life, what would you do?
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    The amazing thing about the default mode network (and the point of this book) is that its activity increases when we are doing nothing. What exactly does this mean? From the perspective of a brain imaging scientist using fMRI, it means that activity in this network spikes when subjects are just lying in a scanner doing nothing.
    More blood is delivering oxygen to the default mode network. More glucose and other brain metabolites are being consumed by this network. And the activity in each region of the network becomes correlated. Scientists can measure how well information is flowing in your default mode network using what’s called “graph theory.”
    Graph theory is a branch of mathematics that was invented in the 18 th century. Recently, it has been remarkably useful in analyzing all kinds of complex networks, especially the brain.
    Networks are made up of nodes. The nodes are connected by things called edges, which are just abstract (or physical) lines drawn between nodes. An edge between two nodes means that there is a relationship between the nodes—i.e., information can flow between these nodes. Sometimes, information can only flow in one direction. This is called a directed edge. In other cases information can flow back and forth between nodes. This is called a non-directed edge. The really useful thing about graph theory is that it can be used to study things as different as air traffic, the internet, and social networks.