Before you start

This is a short book and you can read it in two or three sittings. It's about one habit. You mean to exercise. You have a picture of what a proper session is, which might be forty-five minutes at the gym, a five-kilometre run or a class. On the day, something eats into the time. There's only twenty minutes left, and twenty minutes isn't a proper session, so you do nothing. You'll go properly on Thursday. This happens often enough that the weeks with no exercise in them outnumber the weeks with some.

I'm going to argue three things. The first is that the standard you're holding yourself to is understandable. For years the official advice really did say that exercise only counted in blocks of a certain length. The second is that the standard is wrong, and the advice has changed. The research of the last fifteen years is unusually clear that short amounts of movement are worth having. The biggest health gain on the whole scale comes from the step between nothing and a little. The third is practical. One rule removes most of the zeros from your week. On any day when you can't do the full session, you do ten minutes, and it counts. Chapters 11 to 13 cover how.

A few things this book isn't. It isn't a training plan. It won't get you ready for a marathon, and chapter 10 is plain about what ten minutes can't do. It doesn't argue against long sessions either. If you like your forty-five minutes at the gym, keep them. This book is about what happens on the days when they don't happen. It isn't a weight-loss book. I make no claims about weight at all.

Four cautions.

The NHS advises speaking to your GP first if you haven't exercised for some time, or if you have medical conditions or concerns. That applies to ten minutes as much as to an hour. It also says to make sure the activity and its intensity suit your fitness.

If you get chest pain, stop. The NHS says to call 999 if you have sudden chest pain or discomfort that doesn't go away, pain that spreads to your arm, neck, jaw, stomach or back, or chest pain with sweating, sickness, light-headedness or shortness of breath. Chest pain that comes and goes still needs medical advice.

If you're pregnant, have recently had a baby, or live with a long-term condition or disability, the general message of this book still holds, but the details differ. The NHS and the World Health Organization both publish guidance for these groups, and your GP, midwife or physiotherapist can advise you.

And if your difficulty runs the other way, this book isn't for you. If you feel compelled to exercise, get anxious when you can't, or use exercise to make up for eating, please talk to a GP. The NHS lists exercising too much among the features of some eating disorders, and the charity Beat offers support.

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