Few pieces of health advice are repeated as confidently, or sourced as poorly, as the instruction to drink eight glasses of water a day. There is no study behind that number. What there is instead is a reasonably well understood system that mostly manages itself, and a small number of situations where it needs help.
Thirst is a better guide than a target
The body senses the concentration of the blood and adjusts, releasing a hormone that tells the kidneys to conserve water and producing thirst well before any harm occurs. For a healthy adult with ordinary access to fluid, drinking to thirst covers it. The people for whom that is less reliable are older adults, in whom the thirst signal genuinely does blunt with age, and anyone whose circumstances mask it.
Everything counts, not just water
Total intake includes tea, coffee, milk, soup and the water in food, which contributes a meaningful share. Coffee and tea in normal amounts are mildly diuretic but still net positive for fluid balance — the widespread belief that they dehydrate you does not survive contact with the research.
The one indicator worth glancing at
Urine colour is the practical check: pale straw suggests you are fine, consistently dark suggests you are behind. It is imperfect — B vitamins and some medicines colour it regardless — but it is more useful than counting glasses, and it costs nothing.
When the need genuinely rises
Heat, exercise, fever, vomiting and diarrhoea all increase losses, sometimes sharply. For ordinary exercise under an hour, water is sufficient; the sports drinks are aimed at longer or hotter efforts where salt losses matter. Alcohol suppresses the conserving hormone, which is a large part of why a heavy evening produces a dry, thick-headed morning.
More is not automatically better
Drinking far beyond thirst is not a virtue, and in rare cases — typically endurance events where people drink heavily and steadily — it dilutes blood sodium enough to become dangerous. The sensible position is unglamorous: drink when thirsty, a bit more when it is hot or you are working hard, and stop treating a number as a target.
The kidneys, briefly
They filter continuously and quietly, and early kidney trouble produces almost no symptoms — which is why it is picked up on a blood or urine test rather than noticed. Blood pressure and blood sugar are the two things that damage them most over the years, so keeping those in range is a bigger favour to your kidneys than any amount of extra water.
When to get it looked at
Passing much more or much less urine than usual, blood in the urine, persistent swelling in the ankles, foamy urine, or pain in the flank are all worth an appointment rather than a bigger bottle.
For most people, most of the time, the system is already doing its job.