Introduction
Imagine you spend a full night in bed.
But every twenty minutes, a noise wakes you up: a door slamming, a dog barking, a passing siren.
Did you get a good, restorative night?
Obviously not.
What matters isn't time spent in bed, but how much truly useful sleep you got, especially deep sleep and REM.
Now translate that to a workday. You're at your computer, but every twenty minutes an interruption shows up: an email notification, an instant message, a call, a coworker checking if you saw their message.
The logic is exactly the same.
What matters isn't time spent in front of a screen, but how much truly useful work you do.
An environment built for interruption
A few decades ago, office work happened in very different conditions: a private office, few communication tools, a phone that rarely rang, and written exchanges that were costly in time and energy.
Today, everything changed.
Open offices multiply visual and sound interruptions. Computers and phones let you jump from one task to another in one click. Instant messages and email made communication almost free, which mechanically exploded volume.
On top of that, we now have massive media and digital content production. Information isn't rare anymore. Attention is.
In that context, an office worker, on average, works only a few minutes before being interrupted again. This constant fragmentation creates the illusion of activity, but in reality it destroys focus.
Carlson's law and the multitasking illusion
This reality was formalized as early as the 1950s by Swedish researcher Sune Carlson.
Carlson's law is simple:
a task done continuously takes less time and less energy than the same task done in several fragmented sessions.
Doing A then B is always more efficient than trying to do A and B in parallel.
When your brain switches tasks, it doesn't instantly jump from one topic to another. It needs time to recover its initial attention level. A thirty-second interruption never costs thirty seconds. It costs those thirty seconds plus all the time it takes to refocus.
It's like being woken up in deep sleep: your brain needs time to get back to an optimal state.
The cognitive cost of interruptions
Beyond wasted time, task switching has a direct impact on cognitive ability.
Research in cognitive psychology shows that working while being constantly interrupted significantly degrades mental performance. The focus drop can be comparable to sleep deprivation.
Truly productive people aren't the ones who respond the fastest. They're the ones who stay fully present in what they're doing. In a meeting, they're engaged. In a work session, they don't check messages every few minutes.
This ability to stay locked in is sometimes described as an intense focus state, close to what we cover in flow state and the stretch zone.
Reactivity and productivity are opposites
It's tempting to confuse reactivity with effectiveness. Replying immediately feels useful and available.
In reality, constant reactivity is often a symptom of missing focus. Each immediate reply is a micro-interruption that fragments attention and prevents progress on important tasks.
That's why reducing tool-driven interruptions is a major lever, as explained in remove notifications.
But not all interruptions come from outside. Some are self-generated, which we cover in resisting temptations. Others come from your social environment and require isolation strategies, as explained in create your focus bubble.
What Carlson's law sets up for the rest
Carlson's law sets a fundamental principle: once you start a task, you should ideally carry it to completion without interruption.
The goal of this chapter is to give you the keys to protect your attention, reduce distractions, and create working conditions that support deep, sustainable focus.
From that base, you'll be able to work more calmly, more effectively, and with much less mental fatigue.
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