This two-part 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. Those projects required some plain old-fashioned stick-to-itiveness.

But the insight that led to them probably came from the daydreaming mode. This brain state, marked by the flow of connections among disparate ideas and thoughts, 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 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.

A third component of the attentional system, the attentional filter, helps to orient our attention, to tell us what to pay attention to and what we can safely ignore. This undoubtedly evolved to alert us to predators and other dangerous situations. The constant flow of information from Twitter, Facebook, Vine, Instagram, text messages and the like engages that system, and we find ourselves not sustaining attention on any one thing for very long — the curse of the information age.

My collaborator Vinod Menon, a professor of neuroscience at Stanford, and I showed that the switch between daydreaming and attention is controlled in a part of the brain called the insula, an important structure about an inch or so beneath the surface of the top of your skull. 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 seesaw, then the insula — the attentional switch — is like an adult holding one side down so that the other stays up in the air. The efficacy of this switch varies from person to person, in some functioning smoothly, in others rather rusty. But switch it does, and if it is called upon to switch too often, we feel tired and a bit dizzy, as though we were seesawing too rapidly.

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.