Quick Answer The "8 glasses a day" rule was never a research finding — it originated from a 1945 recommendation that was widely misquoted. How much water you actually need depends on your body weight, activity level, and climate. Most adults are mildly dehydrated for most of the day without knowing it, because thirst only activates after a 1–2% fluid deficit is already established — the same level at which mood, concentration, and physical performance are measurably impaired in controlled tr...