Before you start

This is a short book and you can read it in two sittings. It's about a habit so ordinary that it barely counts as one. You get up, you have a coffee, you have another at your desk, and then you have nothing much until the evening. Somewhere after lunch you go flat, and a headache arrives, and you reach for a third coffee or a biscuit or both, and the flatness lifts a little and comes back. Your pee is dark yellow most of the day. You've noticed that, and then you've stopped noticing.

I'm going to argue three things. The first is that the habit makes sense from the inside. Water is dull, thirst is a quiet signal that's easy to talk over, coffee is what the day has to hand, and the flat feeling at three doesn't announce what it is. The second is that the gap matters more than it looks, and there's research on why. In laboratory studies, losing as little as one or two per cent of your body's water made people more tired, more headachy, less able to concentrate and lower in mood, while barely touching how well they could think. Thirst runs steadily all day while hunger comes in two waves, and most of what people drink is drunk around meals. And the coffee and the biscuit at three are relief for a state that a glass of water would have addressed, which is why they keep being needed. The third is practical. A glass of water with every hot drink and every meal, and a glass first when the afternoon goes flat, checked once a day by the colour of your pee. That's all of it, and chapters 16 to 19 cover how.

The book doesn't tell you to give up coffee, carry a two-litre bottle, or drink eight glasses. Chapter 5 is about where that number came from, and chapter 13 is about the bottle. The NHS says six to eight cups or glasses of fluid a day, that tea and coffee count, and that pale yellow pee is the sign you're getting enough. I'm not going to add to that.

Three cautions.

If you've been sick or had diarrhoea, or you've been in the heat, or you feel dizzy when you stand up and it doesn't pass, that's not a dry day, that's dehydration, and the NHS says to ask for an urgent GP appointment or call 111. Chapter 20 has the full list.

If you have a heart or kidney condition and you've been told to limit how much you drink, follow that advice and not this book.

And if you're older, or looking after someone who is, thirst gets less reliable with age. Chapter 20 covers that too.

The notes at the back give my sources and their limits. Where I'm giving my own reasoning and not a finding, I'll say so.