"The afternoon is just like that"

The heaviness after lunch, the headache, the screen that's harder to read at three than it was at eleven. Everyone gets that, you've been told, and it's the body's rhythm, or the lunch, or the meeting. It's just the afternoon.

Part of it is. There's a natural dip in alertness in the early afternoon that people get whether or not they've drunk anything, and a book that blamed the whole thing on water would be lying. But two studies from the same laboratory measured what mild dehydration on its own does, and the list is familiar.

Lawrence Armstrong, Matthew Ganio and colleagues had 25 young women spend three eight-hour days in a laboratory. On two of the days they were made mildly dehydrated through exercise without overheating, losing on average about one and a third per cent of their body mass in water. On the third day they were kept hydrated. Mood, thinking and symptoms were measured at rest and during exercise. Dehydration made them feel less vigorous and more tired, raised their total mood disturbance score, and made tasks feel harder, made concentration feel harder, and produced headache. Most measures of how well they actually thought weren't affected. In a companion study, Ganio, Armstrong and colleagues did the same with 26 young men, at about one and a half per cent, and found fatigue and tension rose at rest, fatigue rose during exercise, and two specific measures of attention and working memory got slightly worse.

One and a half per cent of body mass for a person of 70 kilograms is about a litre. That's a plausible gap for someone who's had two coffees and a can since seven in the morning and it's now three in the afternoon, particularly in a warm office. I want to be careful here. These studies produced dehydration through exercise in a laboratory, not through sitting at a desk, and nobody has measured your Tuesday. What they show is that the level of dryness a dry day can reach is enough, on its own, to produce tiredness, headache, difficulty concentrating and low mood, while leaving your ability to think mostly intact. That's the flat hour, described from the outside.

So the afternoon is partly just like that, and partly a state you're contributing to. You can't do much about the first part. The second part is a glass of water, and you'll know within a fortnight how much of your afternoon it was.