The human brain evolved to focus on one thing at a time. This enabled our ancestors to hunt animals, to create and fashion tools, and to protect their clan from predators or invading neighbors. In parallel, an attentional filter evolved to help us to stay on task, letting through only information that was important enough to deserve disrupting our train of thought. This filter is what captures your attention when you hear someone mention your name in a crowded room even if you aren’t aware of what they were saying a moment earlier — the system is monitoring events at a pre-conscious level in order to promote some events to consciousness and ignore others. This attentional system is one of the crowning achievements of the human brain, and the focus it enables allowed us to harness fire, build the pyramids, discover penicillin and decode the entire human genome.

But a funny thing happened on the way to the twenty-first century: The plethora of information and the technologies that serve our brain changed the way we use it. Increasingly, we demand that our attentional system try to focus on several things at once, something that it didn’t evolve to do. Multitasking is the enemy of a focused attentional system. We talk on the phone while we’re driving, listening to the radio, looking for a parking place, mentally planning our mom’s birthday party, trying to avoid the road construction signs, and thinking about what’s for lunch. We can’t truly think about or attend to all these things at once, so our brains flit from one to the other, each time with a neurobiological switching cost. The system does not function well that way. Once on a task, our brains function best if we stick to that task.

To pay attention to one thing means that we don’t pay attention to something else. Attention is a limited-capacity resource.

Uni-tasking is getting harder and harder to do. The information age now buries us in data coming at us from every which way. We are bombarded with more information than at any time in history — the equivalent of 175 newspapers a day, five times as much information as we took in thirty years ago. This deluge of facts, pseudo-facts, newsfeeds, and jibber-jabber tricks our attentional system because it is *new* and our attentional system evolved to be sensitive to novelty, to habituate to anything old. That’s why you barely notice the hum of your refrigerator until a friend comes in and points it out.

Sometimes, when we think we’re paying attention, we just zone out.

Have you ever sat in an airplane or train, just staring out the window with nothing to read, looking at nothing in particular? You might have found that the time passed very pleasantly, with no real memory of what exactly you were looking at, what you were thinking, or for that matter, how much time actually elapsed. You might have had a similar feeling the last time you sat by the ocean or a lake, letting your mind wander, and experiencing the relaxing feeling it induced. In this state, thoughts seem to move seamlessly from one to another. There’s a merging of ideas, visual images, and sounds, of past, present, and future. Thoughts turn inward — loosely connected, stream‑of‑consciousness thoughts so much like the nighttime dream state that we call them daydreams.

This distinctive and special brain state is marked by the flow of connections among disparate ideas and thoughts, and a relative lack of barriers between senses and concepts. It also can lead to great creativity. Its discovery — a special brain network that supports a more fluid and nonlinear mode of thinking — was one of the biggest neuroscientific discoveries of the last twenty years. This network exerts a pull on consciousness; it eagerly shifts the brain into mind-wandering when you’re not engaged in a task, and it hijacks your consciousness if the task you’re doing gets boring. It has taken over when you find you’ve been reading several pages in a book (or internet article!) without registering their content, or when you are driving on a long stretch of highway and suddenly realize you haven’t been paying attention to where you are and you missed your exit. It’s the same part that took over when you realized that you had your keys in your hand a minute ago but now you don’t know where they are. Where is your brain when this happens? Well, it’s turned inward, not primarily focused on the external world.

The discovery of this mind-wandering mode didn’t receive big headlines in the popular press, but it has changed the way neuroscientists think about attention. Daydreaming or mind-wandering, we now know, is a natural state of the brain. This accounts for why we feel so refreshed after it, and why vacations and naps can be so restorative. The tendency for this system to take over is so powerful that its discoverer, Marcus Raichle, named it the default mode. This mode is a resting brain state, when your brain is not engaged in a purposeful task, when you’re sitting on a sandy beach or relaxing in your easy chair with a single malt Scotch (Oban, neat, please), and your mind wanders fluidly from topic to topic. It’s not just that you can’t hold on to any one thought from the rolling stream, it’s that no single thought is demanding a response.

The mind-wandering mode stands in stark contrast to the state you’re in when you’re intensely focused on a task; it is responsible for so many high-level things we do that researchers have named that “the central executive.”

These two brain states form a kind of see-saw. When one is up (in terms of neural activation) the other is not; if we’re in one mode we’re not in the other. The job of the central executive is to prevent you from being distracted when you’re engaged in a task, limiting what will enter your consciousness so that you can focus on what you’re doing uninterrupted.

The mind-wandering mode is responsible for our moments of greatest creativity and insight, when we’re able to solve problems that previously seemed unsolvable. You might be going for a walk or grocery shopping or doing something that doesn’t require sustained attention (and the mind-wandering default mode has taken charge of the see-saw) and suddenly — boom — the answer to a problem that had been vexing you suddenly appears. This is the mind-wandering mode, making connections among things that we didn’t previously see as connected.

My collaborator Vinod Menon and I showed that the switch is controlled in a part of the brain called the insula, an important structure about an inch or so beneath the surface of where the temporal lobes and frontal lobes join. Switching between two external objects involves the temporal-parietal junction. If the relationship between the central executive system and the mind-wandering system is like a see-saw, then the insula — the attentional switch — is like an adult holding one side down so that the other stays up in the air.

This efficacy of this network varies from person to person, in some functioning like a well-oiled switch, and in others like a rusty old gate. But switch it does, and if it is called upon to switch too much or too often, we feel tired and a bit dizzy, as though we were see-sawing too rapidly.

Neurons are living cells with a metabolism; they need oxygen and glucose to survive and when they’ve been working hard, we experience fatigue. Asking the brain to shift attention from one activity to another causes the brain to burn up oxygenated glucose, the same fuel needed to stay on task. And the kind of rapid, continual shifting we do with multitasking causes the brain to burn through fuel so quickly that we feel exhausted and disoriented after even a short time. We’ve literally depleted the nutrients in our brain. This leads to compromises in both cognitive and physical performance. Among other things, repeated task switching leads to anxiety, which raises levels of the stress hormone cortisol in the brain, which in turn can lead to aggressive and impulsive behaviors. By contrast, staying on task is controlled by the anterior cingulate and the striatum, and once we engage the central executive mode, staying in that state uses less energy than multitasking and actually reduces the brain’s need for glucose.

Every status update you read on Facebook, every tweet or text message you get from a friend, is competing for resources in your brain with important things like whether to put your savings in stocks or bonds, where you left your passport, or how best to reconcile with a close friend you just had an argument with.

If you want to be more productive and creative, and to have more energy, the science dictates that your social networking should be done during a special designated time, not as constant interruptions to your day.

Keeping your email program active throughout the workday is also distracting, and email should be best done at designated times (an unread email in your inbox can lower your effective IQ by 10 points). And what about increasing creativity? This will happen naturally as we tame the multi-tasking and immerse ourselves in a single task for sustained periods, say 30 – 50 minutes. And as a boost to creativity, several studies have shown that a walk in nature, exercise, or listening to music can trigger the mind-wandering mode. This acts as a neural reset button, and provides much needed perspective on what you’re doing. To work, perchance, to daydream.