Most of us need eight hours a sleep a night to fully function. But the daytime equivalent of needing eight glasses of water per day rests on myth.

Zero glasses per day would leave someone dead within a week, while eight glasses is likely more than necessary, so where does the true number lie? That depends on the person and circumstances.

Whoever the person, their body will be among the least efficient users of water on the planet. Regrettably for us Homo sapiens, the need we can go the second-shortest time without (after oxygen) is one which our bodies can store little of. Further, we have no way to replenish spent water supplies other than drinking it or having it administered intravenously. The latter is impractical outside a medical setting, so we need to make sure we gulp enough, but the idea that means eight glasses a day for persons of every age, weight, climate, and activity level is mistaken.

That notion dates to a 1945 U.S. Food and Nutrition Board suggestion that persons get two and a half liters of water daily. Two key items here. First, this amount was based on the mostly-correct idea that humans on average lose about two liters of water per day. But no research was conducted to affirm the idea of 2.5 liters being right for all persons in all circumstances. Second, the recommendation included the long-forgotten caveat that some of the consumed water could come from food sources.

All foods contain water, from the copious amount in aptly-named watermelon to the negligible level in saltines. The food and drink one intakes without thinking about it may suffice for one’s needs and the easy trick is to let thirst be your guide. There is no need to consciously ingest eight 8-ounce glasses per day unless that happens to coincide with what your thirst dictates.

Humans lose water in vapor form when we breathe and still more is lost through urine and sweat. Even an Inuit couch potato will perspire, though imperceptibly, and this goes back to our inefficient use of internal water supplies. Our bodies use sweat for temperature control by drawing heat off the skin, where it evaporates.

The amount varies by person and environment, but the average amount lost per day to sweating, breathing, and urinating is two liters. Whatever is lost must be replenished to maintain equilibrium. But, again, two liters is merely the average, and the determining factor is how much a given person has lost, and water contained in foods also serves to replace spent reserves.

Of course, one should adjust if in hot weather or doing hard labor. And in an article on the McGill University website, Dr. Christopher Labos cautions that the thirst reflex wanes with age, which is one reason seniors die during heat waves. So age, temperature, and activity can all result in reasonable exceptions to the notion that consciously drinking a set amount of water per day is unnecessary.

If a person in those circumstances drink too much, they should be fine. Except in extreme cases, drinking more water than what the body needs is harmless, though without benefit. Excess amounts will be pissed away. The kidneys’ primary role is to ensure water losses equal water intake. If they fail in this mission and water retention occurs, the victim will experience swollen feet, with this ballooning then creeping its way up the legs. This is nature’s way of letting us know a vital organ is failing and we need medical attention immediately.

There have been isolated cases of water toxemia, a disruption of brain function that occurs when the usual balance of electrolytes is thrown off through severe over-hydration.

The campy 1970s phenomenon, the Book of Lists, reported on a woman who was convinced she was susceptible to the same type of cancer that killed her mother, so she consumed gallons of water for days on end, causing her overtaxed kidneys to shut down, killing her. Then in 2007, Californian Jennifer Strange died in a radio stunt gone horribly wrong. She chugged about two gallons per day without urinating in an attempt to win a contest prize of a Wii system.

Like the 1970s victim referenced in the previous paragraph, some persons think extra water will make the kidneys more efficient. But Labos cited a randomized study in the American Medical Association Journal in which 631 kidney disease patients drank more water than members of a control group and experienced no improvement.

So  the best available evidence points to the notion of needing eight glasses a day to be unfounded. If they start messing with my eight hours of sleep, then we’ll have issues.