"I'm tired, so I should rest"

At three in the afternoon your energy drops. At half past five you get home feeling finished. The obvious reading is that you've used up the day's supply, and the obvious response is to sit down and recover. It's what you'd tell anyone else to do.

This makes complete sense for one kind of tiredness. If you've spent the day laying paving slabs, carrying a toddler round a zoo or walking twelve miles, your muscles have done real work, and rest is what restores them. Bodies that have been used need rest.

But look at what your body did on that Wednesday. It sat. Your mind may have worked hard, and I don't dismiss that. Concentrating for hours, dealing with people and making decisions is tiring in its own way. But the tiredness you feel at half past five doesn't come from physical work, because there wasn't any. Your legs aren't short of rest. They've had about thirteen hours of it.

So you're applying the cure for one kind of tiredness to a different kind, and it doesn't work. You've probably noticed this without putting it into words. An evening on the sofa after a desk day doesn't leave you refreshed. You go to bed as flat as you were at six, and often flatter.

There's an old experiment on this that I like. In the 1980s the psychologist Robert Thayer asked eighteen volunteers to try two things on different days, in the course of their normal lives. On some days they ate a chocolate bar. On others they walked briskly for ten minutes. Each time, they rated their energy, tiredness and tension over the next two hours. After the ten-minute walk they reported more energy and less tension, and the effect was still there two hours later. After the snack they felt more energetic at first, but an hour later they were more tired than before they'd eaten it, and more tense.

Eighteen people rating their own feelings is a small study, and I wouldn't build a whole argument on it. It does match something you can test for yourself in an afternoon, and it points the opposite way from common sense. The thing that feels like spending energy left people with more of it, and the thing that feels like topping up left them with less.

Larger reviews point the same way, with a caveat I should give you. Timothy Puetz and colleagues pooled the trials in which inactive people were put on regular exercise programmes and asked about their energy. Across all of them, people on the programmes reported more energy and less fatigue than people in the comparison groups. But in the subset of trials that used a placebo comparison, such as a group given some harmless alternative activity and the same attention, the exercise groups did no better. So some of the lift people feel may come from expecting to feel better, or from the fact of doing something at all. For your purposes that may not matter much. It does mean I can't tell you that the effect is pure physiology.

The NHS takes the ordinary version of this for granted. Its page on tiredness lists common causes, and not getting much exercise is there alongside poor sleep and stress.

Here's how I think the pieces fit, and this is interpretation. Your sense of energy isn't a fuel gauge. It's closer to a setting that your body adjusts according to what's been asked of it lately. A body that's asked to move every day keeps itself ready to move, and readiness to move is a large part of what feeling energetic means. A body that's asked for nothing turns the setting down. The three o'clock slump, on this view, isn't a sign that you've done too much. It comes partly from having sat since half past eight, along with whatever your lunch and your sleep are contributing.

This gives you a trap with two sides. You feel tired, so you sit. Sitting asks nothing of your body, so it prepares for nothing, and you feel more tired. After a few years the tiredness seems like a fixed fact about you, and you explain it with the only explanation available, which is your age.

None of this means you should never rest, or that a sit-down after work is a mistake. It means that when you've been still all day, more stillness won't fix the tiredness you've got. Next time the slump arrives at three, try ten minutes of brisk walking before you reach for the tea and the drawer. Go round the building or the block. Then see how four o'clock feels.