You've got a carefully managed calendar, a ninja-level to-do list, the best productivity apps available, and maybe even an ace assistant. So why do your days keep spinning out of control and your actual commitments quickly diverge from what you had planned?

Before you gat angry at your tech (or your assistant) for your missed deadlines, consider another possible culprit: your brain. Psychologists tell us that we’re hardwired to be terrible at planning our schedules and estimating how much time tasks will take.

Brain, 1: Rationality, 0

For research recently reported on the British Psychological Society's Occupational Digest scientists out of the University of Oslo in Norway asked students to estimate how long it would take them to complete both small and large reading tasks. You might think rationality dictates that if it takes you one minute to read a single page, it will take you 10 minutes to read 10, but the students didn't see it that way. Instead they guessed smaller tasks would take longer than they actually do, and expressed overconfidence about how quickly they could finish larger projects.

Fine, you might respond, if we're overconfident about how long it takes to complete big tasks, we can just break large projects down into smaller component chunks to avoid the problem. That's exactly what's often done when teams face big projects in commercial settings. It may sound like a sensible plan, but when the researchers studied this strategy, speaking to 94 IT pros who had tackled a large software project by breaking it into 10 pieces or "user stories," they found our brains are not so easily fooled.

Brain, 2: Rationality, 0.

The techies were better than the students at estimating how long specific tasks would take, but that didn't mean that dividing a project into smaller pieces guaranteed it would be accomplished in on time. To find out why, the researchers flipped the question, asking the developers not "how long will it take you to complete X," but instead "if I give you X amount of time, how much will you get done?"

When asked to estimate how much someone would get done in 20 hours, the IT pros predicted a hypothetical developer would be much more productive per hour than when they were asked the same question about a 100-hour period. This meant that when the project was broken down into smaller segments, the techies thought: "I can get X done in 20 hours, so in 100 hours I should be able to get 5X done," only to discover that they'd been significantly overconfident about their productivity over short periods. The result: a lot of late nights burning the midnight oil. Or missed deadlines.

How Can We Even the Score?

If we know our brains are conspiring against our best efforts to manage our schedules, what's to be done? BPS sums up the takeaway from the research nicely:

These studies suggest "smaller magnitudes (of work or of time) are judged as disproportionately larger than large magnitudes." Breaking a software project down into a quick succession of releases may encourage unrealistic estimates of just how much of the project will get done in each release. Therefore, it's valuable to reverse your thinking and focus on the sub-tasks involved, and sense-check whether their durations really do fit your fixed deadline.

Or to put it another way, don't be shy about checking your brain's natural tendency to mess up your schedule. Try using a timer—and a bit of back-of-the-envelope math.

Do your estimates of how long it will take to do something often fail to match up to reality? What strategies do you use to keep your schedule in check?