The symptom you blame on everything else
Mild dehydration does not feel like thirst. It feels like other things: a flatness in the afternoon, a headache with no obvious author, a short temper you blame on the inbox. Fatigue, headache and irritability are its signature, and because those three symptoms have a hundred possible causes, water is usually the last suspect interviewed.
People reach for coffee, then sugar, then a lie-down, and the actual deficit, a glass or two of fluid, goes unexamined. The fix costs nothing and takes thirty seconds, which may be exactly why nobody respects it.
Why thirst is a late signal
The thirst mechanism is a lagging indicator. By the time it fires, the effects it exists to warn about have already begun: concentration dips, patience shortens, the headache clears its throat. Waiting for thirst is like waiting for the smoke alarm to tell you dinner is done.
This is why "I drink when I am thirsty" fails as a system for most people. The signal arrives after the consequences, every day, on schedule. You cannot fix a lagging indicator by listening to it harder. You fix it by getting ahead of it, which is a question of timing rather than quantity. A glass of water on a schedule beats a pint of water in a panic.
Timing beats volume
The instinctive fix is volume: a heroic litre at noon, another at six. It does not hold. A large amount taken at once passes through quickly, and the hours either side of it run as dry as before. Small amounts spread across the day hold a much steadier line. A glass on waking. One mid-morning. One with lunch, one mid-afternoon, one with dinner.
The total is often less than the heroic version. The coverage is better. Two large glasses do not cover ten hours. Your body cares less about how much water arrived in a day than about the shape of the day it arrived in.
A routine you will not think about
The routine that survives is the one with no decisions in it. A glass on waking, before anything else happens. A glass with each meal, because the meal already happens. A bottle within reach at your desk, because distance is the silent killer of good intentions, and the water you drink is the water that is near.
That is the entire system: fixed points and a bottle. Nothing to log, nothing to optimise. After a fortnight it runs itself, and the afternoon flatness quietly stops filing reports.
Boring advice, held daily, beats brilliant advice held twice. Drink a glass now, one with lunch, and let the routine be the dullest thing you own.