Why you make more mistakes when your head is full

When your head is full, it stops choosing and just reacts. Filling it up is like asking the autopilot to take over. And changing that is actually simple.

We make more mistakes with a full head because the memory we use to think is small, and every pending task takes up a piece of it. By the end of the day, with little free space and tiredness piling up, the brain starts deciding on autopilot, and that’s when the slip-ups show up.

We start the day with more energy. Morning is usually the best time to make decisions.

At night, we’re more worn out, with lots of tasks piled up in our heads. It’s like a phone with too many apps open. Even simple tasks start to freeze, and everything slows down.

It’s like a full glass. Any extra drop feels like too much. We spend the whole day piling up things to solve, until something tiny can knock anyone over.

How much can your head hold at once? What science says

The image of the phone with open apps is more literal than it seems. The part of memory we use to hold information while we think is quite small. A widely cited review by psychologist Nelson Cowan estimates that we can keep only about four “chunks” of information active at the same time.¹ Every to-do left running in your head takes one of those slots, leaving less room to think about what’s right in front of you.

The day’s fatigue also shows up in the numbers. Researchers from Denmark and the United States analyzed the national tests of more than two million Danish students. For every hour later in the day the test was taken, scores dropped a little. When students had a 20 to 30 minute break before the test, the drop not only disappeared, scores actually improved.² The mind gets tired, and it also recovers quickly when it gets a break.

There’s also a finding that helps a lot if you live with a to-do list in your head. In a series of experiments, psychologists E. J. Masicampo and Roy Baumeister asked people to think about an important task they hadn’t finished yet. That hurt their performance on other activities, because the unfinished task kept popping back into their thoughts. But when participants wrote down a simple plan for when and how they would handle it, the interference practically vanished.³ The task was still undone. What changed was that the brain no longer needed to keep reminding them of it.

How do you make fewer mistakes with a full head? Pay attention to the details

That’s why we need to be smart about not letting our heads fill up with things that steal our attention or energy. Knowing how to prioritize the most important tasks and doing them at the best time makes all the difference.

The first thing to think about is handling what really matters as efficiently as possible. During the day, every task you finish is one less taking up space in your head when night comes.

At that point, think about the to-dos that bring the most results, the ones that can’t be postponed and the truly important ones. Bills due today, necessary conversations, immediate commitments.

And whatever can’t be handled right now deserves at least to go on paper, with a day and a time. It can be a notebook, a to-do app or a sticky note on the fridge. It seems like a small thing, but that’s what takes the to-do out of your memory and gives that slot back to what you’re doing.

Where the key to success really turns

If you get organized for that first batch of to-dos, a big part of the day is already handled. In many cases, getting those few main tasks right already covers most of what the day needed to deliver.

The other tasks, which aren’t for today or aren’t that important, can wait. What matters is that this is a conscious choice. Everything in its own time.

It’s also worth respecting the clock. If possible, leave the decisions that need more care for the part of the day when you’re most rested, and save the late afternoon for more mechanical work. And when you notice you’re making silly mistakes, treat it as a warning that the glass is full. Sometimes ten minutes away from the screen solve more than an hour of pushing through.

Getting organized this way, taking priorities and time of day into account, the mind tends to work better. Organizing this list is like paving the road so your brain can race with less effort.

Something to think about

So let me ask you: how’s the pavement on your road?

To improve your pavement, here are three more reads:

• Good pavement starts the night before, and here’s what you lose when you cut your sleep.
• To tune up the car, nothing beats muscles and a healthy routine. See how exercise changes your brain before your body.
• When the list doesn’t fit in your head, the one task rule helps you choose where to start.

Editor’s note: We all have that coworker who, after 5 p.m., starts making decisions with the clarity of someone driving blindfolded. Send them this link. Maybe they’ll realize the problem isn’t the universe conspiring against them, it’s just that their brain’s road urgently needs some focus.

Footnotes

1. Cowan, N. (2001). The magical number 4 in short-term memory: A reconsideration of mental storage capacity. Behavioral and Brain Sciences, 24(1), 87-114. ↩ source

2. Sievertsen, H. H., Gino, F. & Piovesan, M. (2016). Cognitive fatigue influences students’ performance on standardized tests. Proceedings of the National Academy of Sciences, 113(10), 2621-2624. ↩ source

3. Masicampo, E. J. & Baumeister, R. F. (2011). Consider it done! Plan making can eliminate the cognitive effects of unfulfilled goals. Journal of Personality and Social Psychology, 101(4), 667-683. ↩ source