"I wouldn't feel any better for it"

Set aside how long you live. Most people exercise, when they do, because of how it makes them feel that day. It clears the head and takes the edge off a bad afternoon, and they sleep better. A common thought at twenty to seven is that a short session wouldn't deliver any of that. You need the full forty-five minutes to get the lift.

Three researchers, Hansen, Stevens and Coast, tested this. They had people come in on four occasions. On one occasion they rested quietly. On the others they rode an exercise bike at a moderate intensity for ten minutes, twenty minutes or thirty minutes. Before and after each, they filled in a standard mood questionnaire. Exercise improved vigour and reduced fatigue and overall negative mood. Those improvements were present after ten minutes. One measure, mental confusion, went on improving up to twenty minutes. Beyond twenty minutes there was no further improvement in mood at all. The thirty-minute session didn't leave people feeling any better than the twenty-minute one, and on most measures no better than the ten.

It's one study, done on an exercise bike in a laboratory, and I wouldn't build a cathedral on it. But it agrees with common experience once you look for it. Think of the last time you walked briskly to the station because you were late. You arrived warm and awake and in a slightly better mood. That took eight minutes and you weren't even trying to exercise.

There's a second finding that bears on the moment of decision. Matthew Ruby, Elizabeth Dunn and colleagues ran four studies with 279 adults. Some people predicted how much they'd enjoy an exercise session they were about to do. Others reported how much they'd enjoyed it once it was over. People consistently underestimated how much they would enjoy it. This held for group classes and solo sessions, for moderate and hard sessions, and for yoga, Pilates, aerobics and weights.

The researchers also worked out why. When people imagine a session, they give too much weight to the beginning of it, and the beginning is the worst part. You feel stiff and cold and reluctant, and for the first few minutes everything feels like an effort. Then the body warms up and it gets better, and the end is often pleasant. But the forecast is dominated by those first few minutes. The researchers call this forecasting myopia. When they got people to think about the middle of the session instead of the start, predicted enjoyment went up, and so did people's intention to exercise.

Apply this to the hall on Tuesday. You're tired, you've had a bad journey and dinner needs making. You ask yourself whether fifteen minutes of exercise would be worth it, and your mind supplies a picture. It's you, cold and stiff, starting. It doesn't show you at minute nine, warmed up and breathing well, with the meeting finally out of your head. So the answer comes back no. It came from a forecast, and this kind of forecast is known to be too gloomy.

The objection is fair. "Sometimes I'm exhausted and I wouldn't enjoy it." That's true sometimes. But the person in the hall at twenty to seven isn't well placed to judge. The most reliable way to find out is to begin, and to give yourself leave to stop after ten minutes if it's as bad as you feared. Most of the time it won't be, and you'll feel better for it that evening.