Multitasking has been found to increase the production of the stress hormone cortisol as well as the fight-or-flight hormone adrenaline, which can overstimulate your brain and cause mental fog or scrambled thinking. Multitasking creates a dopamine-addiction feedback loop, effectively rewarding the brain for losing focus and for constantly searching for external stimulation. To make matters worse, the prefrontal cortex has a novelty bias, meaning that its attention can be easily hijacked by something new – the proverbial shiny objects we use to entice infants, puppies, and kittens.

In a famous experiment, my McGill colleagues Peter Milner and James Olds, both neuroscientists, placed a small electrode in the brains of rats, in a small structure of the limbic system called the nucleus accumbens. This structure regulates dopamine production and is the region that “lights up” when gamblers win a bet, drug addicts take cocaine, or people have orgasms – Olds and Milner called it the pleasure centre. A lever in the cage allowed the rats to send a small electrical signal directly to their nucleus accumbens. Do you think they liked it? Boy how they did! They liked it so much that they did nothing else. They forgot all about eating and sleeping. Long after they were hungry, they ignored tasty food if they had a chance to press that little chrome bar; they even ignored the opportunity for sex. The rats just pressed the lever over and over again, until they died of starvation and exhaustion. Does that remind you of anything? A 30-year-old man died in Guangzhou (China) after playing video games continuously for three days. Another man died in Daegu (Korea) after playing video games almost continuously for 50 hours, stopped only by his going into cardiac arrest. Each time we dispatch an email in one way or another, we feel a sense of accomplishment, and our brain gets a dollop of reward hormones telling us we accomplished something. Each time we check a Twitter feed or Facebook update, we encounter something novel and feel more connected socially (in a kind of weird, impersonal cyber way) and get another dollop of reward hormones. But remember, it is the dumb, novelty-seeking portion of the brain driving the limbic system that induces this feeling of pleasure, not the planning, scheduling, higher-level thought centres in the prefrontal cortex. Make no mistake: email-, Facebook- and Twitter-checking constitute a neural addiction.

Although we think we’re doing several things at once, multitasking, this is a powerful and diabolical illusion. Earl Miller, a neuroscientist at MIT and one of the world experts on divided attention, says that our brains are “not wired to multitask well… When people think they’re multitasking, they’re actually just switching from one task to another very rapidly. And every time they do, there’s a cognitive cost in doing so.”

To make matters worse, lots of multitasking requires decision-making: Do I answer this text message or ignore it? How do I respond to this? How do I file this email? Do I continue what I’m working on now or take a break? It turns out that decision-making is also very hard on your neural resources and that little decisions appear to take up as much energy as big ones. One of the first things we lose is impulse control. This rapidly spirals into a depleted state in which, after making lots of insignificant decisions, we can end up making truly bad decisions about something important. Why would anyone want to add to their daily weight of information processing by trying to multitask?

Our brains are busier than ever before. We’re assaulted with facts, pseudo facts, jibber-jabber, and rumour, all posing as information. Trying to figure out what you need to know and what you can ignore is exhausting. At the same time, we are all doing more. Thirty years ago, travel agents made our airline and rail reservations, salespeople helped us find what we were looking for in shops, and professional typists or secretaries helped busy people with their correspondence. Now we do most of those things ourselves. We are doing the jobs of 10 different people while still trying to keep up with our lives, our children and parents, our friends, our careers, our hobbies, and our favourite TV shows.

Just having the opportunity to multitask is detrimental to cognitive performance. Glenn Wilson, former visiting professor of psychology at Gresham College, London, calls it info-mania. His research found that being in a situation where you are trying to concentrate on a task, and an email is sitting unread in your inbox, can reduce your effective IQ by 10 points. And although people ascribe many benefits to marijuana, including enhanced creativity and reduced pain and stress, it is well documented that its chief ingredient, cannabinol, activates dedicated cannabinol receptors in the brain and interferes profoundly with memory and with our ability to concentrate on several things at once. Wilson showed that the cognitive losses from multitasking are even greater than the cognitive losses from pot‑smoking.