"Once I've worked it out, I'll sleep"

This is the promise that keeps you at it. The thinking feels like the obstacle between you and sleep. If you can just settle the question of the roof, your mind will be satisfied and will let you go. So you try to think your way through to the other side.

It doesn't work, and there are two reasons.

The first is that there's always another item. You may have noticed that on the rare occasions when you do reach some sort of resolution on the roof, your mind doesn't then go quiet. It moves on to your mother's hip. If you settle that, it moves to the email. There are always more items. Anyone with a job, a family and a house has an effectively endless supply of unfinished business, and the night shift will work through as much of it as you give it time for. Going back to sleep doesn't depend on clearing the list, since the list can't be cleared.

The second reason is more basic. Thinking hard about problems is the kind of activity that keeps a person awake.

Falling asleep requires a particular state. The body is calm, the mind is idling, and attention is loose. Problem-solving produces close to the opposite. Harvey's model, which I mentioned in chapter 2, describes the sequence. Worried thinking in bed triggers bodily arousal and emotional distress. You'll recognise the signs, which are a faster heartbeat, tense shoulders and a feeling of being switched on. That state in turn makes you watchful for threats, including the threat of not sleeping, which feeds more worried thinking. It goes round in a loop, and every part of the loop keeps you awake.

So the night shift has things backwards. It treats mental work as the route to sleep, when mental work is what's blocking it. Every sum you do about the roof and every imagined argument with your sister makes your body a little more alert. You're trying to get to sleep by doing the thing most likely to keep you awake.

This is also why the urgency makes it worse. The promise says, "Solve this quickly and you can rest," so you work harder. You feel the minutes going and you press on, and the effort raises your alertness further.

It can help to see where the promise comes from. By day it's often true. If a problem is nagging at you at your desk, dealing with it does bring relief, and your mind does let go. The three o'clock mind applies the same rule in a setting where the rule can't work, because the dealing-with stage isn't available. You can think about the problem, but you can't deal with it. So the relief never arrives, and the nagging goes on.

What settles the mind at night is being able to trust that the problem is in hand. Your mind needs to know that the matter has been noted and has a time set aside, and that it will be dealt with by someone competent. That someone is you, tomorrow, after breakfast. Chapter 11 looks at some evidence for this, and chapter 12 is about how to set it up.