"It's quiet, so I can think"

It seems obvious that a silent house in the middle of the night is a good place to think. There are no interruptions and nobody wants anything. You have the problem all to yourself.

The trouble lies with what you're thinking with. Your mind at three in the morning isn't the mind you have at ten, and the difference goes well beyond tiredness.

Mood runs on a daily rhythm, much as body temperature does. Diane Boivin and colleagues showed this in a demanding experiment. Twenty-four healthy young volunteers lived for weeks in a laboratory on artificial days of twenty-eight or thirty hours. Their sleeping and waking drifted out of step with their internal body clocks. This let the researchers separate two things that are normally tangled together, which are how long you've been awake and what time your body thinks it is. Mood varied significantly with the body clock. It wasn't simply a matter of being tired. The same person, equally rested, felt worse at some points of their internal day than at others.

Andrew Tubbs and colleagues pulled the later research together in a review published in 2022, under the title "The Mind After Midnight". They report that positive mood peaks during the day and reaches its lowest point in the night, between about one and four in the morning. Negative mood holds fairly steady through the day and then rises sharply during that same trough. A person awake in the small hours, they write, is awake when positive feeling is lowest and negative feeling highest, and this shapes what they attend to and how they interpret it.

The review goes on to the planning and judging functions of the brain, which depend heavily on the prefrontal cortex, the area behind the forehead. These functions include weighing risks, keeping things in proportion, holding back an impulse and thinking through consequences. The authors gather evidence that they're impaired during wakefulness in the biological night, partly through lost sleep and partly through the body clock itself. They also describe increased activity in the brain's emotional and threat-detecting systems. They suggest that the combination pushes a person towards attending to negative things, giving them too much weight, and ruminating.

I need to be careful here. The review presents a hypothesis. The authors pull together many strands of evidence, some from laboratory studies and some from population records of when things such as violent crime, substance use and suicide occur. They say themselves that the idea needs direct testing. But the separate strands are well supported, and together they describe something you already know from the inside.

So consider what the night shift is. You've taken your hardest problems, the ones involving money, family and work. You've brought them to the one period of the day when you feel worst and your sense of threat is most active. It's also when the part of you that says "hang on, it isn't that bad" is least available. Then you've tried to reach conclusions.

Nobody would choose this in daylight. If a colleague said they did all their financial planning at the end of a sixteen-hour day, in a low mood, after several drinks, you'd suggest another time. The three o'clock mind is in a broadly similar condition, and it's the mind you've been handing the big questions to.

The quiet is real, and it's valuable, and I understand the wish not to waste it. But you're in no state at that hour to make use of it. Chapter 12 looks at how to get some quiet at a time of day when your mind is working properly.