Habit

A habit is a behaviour you've repeated so often in the same situation that it practically runs by itself. Brushing your teeth, reaching for your phone, a glass of juice with breakfast: you no longer decide consciously – your brain plays back a stored programme. In diary studies by the psychologist Wendy Wood, around 43% of daily actions happened this way: almost every day, in the same situation, while people's thoughts were elsewhere.

When it comes to losing weight, that's both a curse and a blessing. Unhelpful habits like crisps in front of the TV or something sweet after lunch run so automatically that you hardly notice them. The flip side: once something has become a habit, it takes hardly any effort. The walk after dinner, the apple in your bag, the glass of water on your desk – after a few weeks to months, all of it feels completely normal.

How long does it take? In a 2010 study at University College London, the psychologist Phillippa Lally found that participants needed a median of 66 days before a new behaviour became largely automatic – with a wide range of 18 to 254 days. So the often-quoted “21 days” isn't backed by evidence. Missing the odd day didn't significantly slow down habit formation.

The best trick is to attach a new habit to an existing one: “After I brush my teeth, I drink a glass of water.” Start so small that you can manage it even on bad days. And shape your environment: fruit out on the table, sweets in the top cupboard. For families this is a particularly effective lever, because children pick up habits from the family table more readily than from advice.