Losing just 1.36 percent of your body weight in water, about 800 milliliters for a 60 kg woman, left young women more tired and less focused. They also got more headaches, in a University of Connecticut study. Twenty-five women around age 23 spent three separate 8-hour days in the lab. On two of those days they became mildly dehydrated through moderate exercise, and on one of them they also took a water pill. On the third day they stayed properly hydrated. Throughout each day, they answered mood questions and took thinking tests, both at rest and while exercising. The study was published in the Journal of Nutrition in 2012. At just 1.36 percent dehydration, the women felt less energetic, more tired, and worse overall on a standard mood questionnaire. They also said tasks felt harder, found it harder to concentrate, and reported more headaches. These effects showed up both at rest and during exercise, yet most scores on the thinking tests held steady, so the damage showed up in how the women felt far more than in how they performed. Part of the dehydration came from exercise and a water pill, a different route from simply forgetting to drink at your desk, and 25 women is a small group. In this study, a shortfall of less than a liter of water was enough to make ordinary tasks feel harder.