"I only do it when I'm tired or stressed"

You may have noticed that the habit isn't steady. On a good morning you lock the door and go. On a bad one, after a poor night or before a hard day, you're back at the handle three times. That seems to you like evidence that the checking is sensible. When you're tired you're more likely to forget, so you check more. What could be more reasonable?

I'd look at it the other way round. What follows is mostly my own reasoning, with Rachman's model behind it.

Go back to the three multipliers from chapter 7. How responsible you feel, how likely the harm seems, how bad it would be. None of them is fixed. All three rise when you're tired, anxious or under pressure, because tiredness and anxiety make every threat feel closer and every duty feel heavier. So on a bad morning the doubt is louder, and the loudness has nothing to do with the door. It's a report on your night. The door is locked in exactly the way it was on Friday, when you didn't go back at all.

There's a second thing that happens on bad mornings. You're distracted, so the once is done on autopilot, without the senses or the sentence. That produces a thin picture, and a thin picture invites the recheck, and the recheck fades it further. So the bad morning gives you a louder doubt and a weaker memory at the same time, and the two together feel like proof that you can't be trusted when tired.

The answer isn't to check more when tired. It's to do the once more carefully when tired, because that's when it's most likely to be skipped. Two seconds longer at the door. The sentence said slowly. Then the same rule as any other day. Leave, and let the doubt be a feeling.

It helps to know in advance which mornings will be bad. After a late night, before an interview, in a difficult week. On those mornings, expect the doubt to be loud and treat its loudness as a mood, the way you'd treat irritability or a headache. "I'm tired, so the doubt is loud today" is a sentence that takes most of the force out of it, because it names the real cause. The alternative, "I'm tired, so I'd better check," takes the loudness as information about the house, and it isn't.

One caution. If stress has made the checking spread quickly and take up real time, that's the moment to reread the first caution in this book. A habit that grows sharply under strain is one worth mentioning to someone.