"I'll just do one more thing"

The estimate is one half of the habit. The last thing is the other, and I think it does more damage.

At twenty to nine you had five minutes in hand. Rather than leave, you answered an email. This felt sensible. Five minutes is enough for a short reply, and leaving early would have wasted them. So the last thing has a slot, the slot is exactly the size of the spare time, and the last thing then takes longer than the slot.

It nearly always does, and I don't think that's bad luck. The last thing is chosen because it looks small, and small tasks are the ones whose steps you haven't pictured. The email had two questions, and the second one needed a figure, and the figure was in a spreadsheet. None of that was in the picture when you sat down. Buehler's finding applies here in miniature. You estimated the email from the plan, the plan was a best case, and the real email had a spreadsheet in it.

Something else happens once you're doing it. When your attention is taken up by a task, time seems to pass more quickly than it does. Richard Block, Peter Hancock and Dan Zakay pooled a hundred and seventeen experiments on this in 2010. When people knew they'd have to judge how long something took, and their attention was loaded with a demanding task, their estimates of the time passing came out short. The harder you're concentrating, the less time you think has gone by. So you sit down to spend five minutes on the email, and when you look up, feeling as though five minutes have passed, it's ten to nine. The clock was in front of you the whole time. You weren't looking at it, because the email was.

I'd add my own reading of why the last thing is so attractive. It converts spare time, which feels like nothing, into a finished job, which feels like something. It also puts off leaving, which for many people is a small unpleasantness, a matter of coats and keys and the cold. And it has a run of successes behind it. On the days when the email really was two lines, you sent it, left, and were on time, and it felt like skill.

There's one piece of research on lateness in particular that bears on this. Jeffrey Conte and Rick Jacobs studied a hundred and eighty-one train operators and measured something called polychronicity, which is how much a person prefers to be doing two or more things at once. The people who scored higher on it were more often late for work. The association was modest, but it's the kind of thing you'd expect if the last thing is part of what makes people late. Someone who likes to have several things on the go doesn't like to stop one to leave.

So the last thing isn't a small extra on top of a sound plan. It's a task with a best-case estimate, done in a window that was already too small, under conditions that make time feel slower than it is. Chapter 17 will ask you to give it up in the final stretch before leaving, and I know that's the hardest thing in this book.