Lack of water is the number one trigger of daytime fatigue. In general, to remain healthy we need to take in enough water to replace the amount we lose daily through excretion, perspiration, and other bodily functions, but that amount can vary widely from person to person, based upon a variety of factors such as age, physical condition, activity level, and climate. The origins of the 8-10 glasses per day figure remain elusive. Drink at least eight 8-ounce glasses of water a day. This unquestioned rule is itself a question mark.
Most nutritionists have no idea where it comes from.
To replace daily losses of water, an average-sized adult with healthy kidneys sitting in a temperate climate needs no more than one liter of fluid, according to Jurgen Schnermann, a kidney physiologist at the National Institutes of Health. One liter is the equivalent of about four 8-ounce glasses. In short, though doctors don't recommend it, many of us could cover our bare-minimum daily water needs without drinking anything during the day. Certainly there are beneficial health effects attendant with being adequately hydrated, and some studies have seemingly demonstrated correlations between such variables as increased water intake and a decreased risk of colon cancer. Not at all, doctors say.