Our brains are not wired for multitasking (working on two or more activities at the same time.) Attempting to do so reduces our energy level, puts a strain on our cognitive resources, increases stress on our body, and possibly damages the memory regions of our brain. In addition, it reduces our personal productivity. And in some cases, it is a high-risk behavior – such as texting while driving.

When we attempt to multitask, we are sharing our attention with two activities, switching back and forth between one task and another. If you monotask, you don’t switch at all if you can complete the task in 90 minutes or less. This is mentioned later. (For a full explanation of why I chose 90 minutes as the ideal length of time to work on tasks and projects, please refer to chapter one of my latest eBook, Time Management for Authors and Writers, published by Bookboon.)

The term “multitasking” was coined by IBM in 1965 to describe how computers worked. It was defined as “the ability of a microprocessor to apparently process several tasks simultaneously”. But computers don’t do that. It just seemed that way because computers are so fast. According to Neil Pasricha, in his book, The Happiness Equation, “Computer multitasking in single-core microprocessors actually involves timesharing the processor; only one task can actually be active at a time, but the tasks are rotated through many times a second.”

Our brains can do the same thing but much slower. And with costly mistakes. For example, texting while driving. If we ignore our driving for two or 3 seconds and focus on the task of texting, then back to driving for a second or so, and then back to texting, at 60 mph, in 3 seconds you would travel 264 feet with an absent driver!

You are also attempting to multitask when you work on a task for an hour or so, switch to e-mail for 20 minutes, then spend a half hour on social media, and then start to work on a different task, and so on. The less time we spend on each task, before switching, the more time it takes to get all your work done.

Dr. Theo Tsaousides, author of the book, Brain Blocks, explains how shifting 100% of your attention back and forth from one task to another requires a tremendous amount of energy and causes mental fatigue. He refers to it more accurately as “serial multitasking.” You need to remember where you were with the original task and transfer 100% of your attention to something else, such as checking e-mail and either responding, deleting, filing, or forwarding messages. You then must regain focus on the main task and put the other second task completely out of your mind. Each time you switch from one task to another, you experience attention residue as well. (You are still thinking about the previous task while you start working on the other one.)

That is one reason I recommend working in 90-minute chunks of time on your important projects. Ninety minutes is a reasonable time to maintain focus, and a reasonable time for would-be interrupters to wait for a reply. (Working in snippets of time in waiting rooms or on airplanes is not very productive. It’s better than nothing, although rest and renewal might be a better option.)

The inefficiency of “serial multitasking” doesn’t end there.  It has been discovered that “retroactive Interference” and “proactive interference” are at play as well. (They are sometimes referred to as “retroactive inhibition” and “proactive Inhibition.) They seem closely related to attention residue but are distinct phenomena.

Retroactive interference is when new information interferes with the ability to recall previously learned information. It is believed to be one of the causes of forgetting. (In addition to neglecting to spend enough time reviewing and repeating the information.) For example, if you are studying for an exam and interrupt yourself to check your e-mail, what you were studying is pushed aside by what you are now doing, and you may have difficulty recalling what you had just learned when you return to it. Frequent interruptions make it worse.

Proactive interference is the opposite. It’s when old information interferes with the ability to retain new information. For example, when I was teaching memory training, everyone successfully memorized a grocery list of 10 items by linking them together in a story. But if I asked them to remember a second list using the same method, many of them mistakenly included an item or two from the first list.

Another example of proactive interference happens when I try to learn Spanish. (My son and his family live in Mexico.) When I review the days of the week in Spanish, my previously memorized high school French interferes, and my Lundi, Mardi, Mercredi etc. get mixed up with my Lunes, Martes, and Miercoles. This problem would be much worse if I were to switch too soon from studying one language to a different one. The same problem might occur when studying or memorizing any information that is somewhat like something you had already learned.

The Zeigarnik Effect can also work against you when you switch from an unfinished task, such as interrupting yourself from a task to check email. This effect was first observed with servers in restaurants. They seemed to remember everything that each guest ordered until they had been served. Then they quickly forgot. Zeigarnik, a Soviet psychologist, studied this phenomenon in the 1920s. Unfinished tasks are more securely held in your short-term memory. So, when you switch to another task before the first one is complete, your brain is using up energy hanging onto the memory of the first unfinished task. It is also related to attention residue, but while the Zeigarnik Effect is about memory, attention residue is about attention lingering on a previous task.

Attempting to multitask facilitates other brain-based phenomena, which also negatively impact your performance, such as FOMO (fear of missing out on something), and “mind wandering.” (They are included in a book I am currently writing on “Always check your email in the morning – and other brain-based strategies.”)

But you get the idea. “Serial multitasking” reduces performance as well. So, try to work on your major tasks for longer periods of time without interruption. The Pomodoro Technique, which suggests that you work on projects in 25-minute chunks of time, followed by 5-minute breaks falls short – literally – since it introduces serial multitasking. I suggest instead the 90-minute chunk of time mentioned earlier, after having cleared your mind of unfinished email, and you have arranged for potential interruptions to be intercepted during that 90-minute chunk of time.

Multitasking can become a habit if you don’t slow down. Hurry is a catalyst for both normal multitasking (attempting to do two tasks at the same time) and serial multitasking (switching back and forth frequently from one task to another.) If you want an example of the former, take another look at the image accompanying this article. The spelling error occurred when I was thinking about something else while I was positioning the letters. “Haste makes waste” s not a useless bromide.