"I need it to get through the afternoon"

Of the three jobs, the pick-me-up seems the most scientific. Sugar is energy. Your blood sugar is low in the afternoon, so you top it up, and the machine runs again. Athletes do it, which is why there are glucose tablets and energy drinks. It sounds like simple fuel management.

It's been tested many times, because researchers were interested in the same question. Konstantinos Mantantzis and colleagues gathered every good study they could find in which people were given carbohydrate, mostly sugar, or a sugar-free placebo, and their mood was measured afterwards. That came to 31 studies with 1,259 participants. They looked at alertness, tiredness, calmness and general mood, and at different time points after eating.

They found no positive effect of the sugar on any aspect of mood at any time point. The effects they did find went the other way. Within the first hour, people who'd had the sugar were more tired and less alert than people who'd had the placebo. The researchers titled their paper with a question, whether there was a sugar rush or a sugar crash, and their answer was that the rush is a myth.

Robert Thayer's small experiment from the 1980s shows what this looks like in daily life. He had eighteen volunteers try two responses to needing a boost, on different days, in the course of their ordinary routines. On some days they ate a chocolate bar. On others they walked briskly for ten minutes. After the chocolate bar they felt more energetic at first. An hour later they were more tired than before they'd eaten it, and they were also more tense. After the walk they felt more energetic and less tense, and they still did two hours later.

So the pick-me-up works like this. You're flagging at quarter past three. You eat the chocolate bar. You feel brighter for a short while, partly from the lift in the last chapter and partly from having stopped work and done something. By four you're lower than you started, which is the dip. The dip feels like proof that you needed sugar, so you think about another coffee and whatever else is in the drawer. You can go round that circuit twice in an afternoon.

I'd add my own reading here, which goes beyond the studies. The belief in sugar as fuel survives because the first ten minutes are the part you connect with the chocolate. The tiredness an hour later gets blamed on the afternoon, the job or your age. Nobody thinks, at four o'clock, that they're tired because of what they ate at quarter past three.

There's a fair objection. Sugar really is fuel for muscles doing hard physical work, and a runner two hours into a race does benefit from it. But at a desk your body isn't short of fuel. You had lunch two hours ago. The flat feeling at three has other sources. It's a natural low point in the day's rhythm for most people, and it's made worse by a poor night, a heavy or sugary lunch, a stuffy room and several hours of sitting still. Eating sugar doesn't address any of those causes.

What does work is dull and well supported. Get up and walk briskly for ten minutes, outside if you can. Drink some water. If you're properly hungry, which sometimes you will be, eat some real food. Chapter 15 goes through the options. For now, on the next afternoon you hit the wall, try Thayer's experiment on yourself. Walk for ten minutes in place of the chocolate bar and then check how you feel at four.