How much water should you drink every day? The latest study from researchers at the University of Utah in Salt Lake City concludes that the old standby of eight glasses a day still holds. Here's the surprising gotcha: If you don't drink enough water, the greatest effect may not be physical--it's mental and emotional.

"For people who aren't exercising a lot or living in a very warm climate, eight glasses of water a day may be a good rule of thumb," lead researcher Dr. Wayne Askew told Reuters. If you do exercise and sweat a lot, then you need more than eight glasses to properly hydrate your body.

Our bodies need water, primarily because water makes up more than 70 percent of solid body tissue. It helps regulate body temperature, carries nutrients and oxygen to cells, removes waste, cushions joints, and protects organs and tissues, explains Reuters. Too little water can cause headaches, grogginess, and dry, itchy skin. When we become severely dehydrated, it can affect our blood pressure, circulation, digestion, kidney function, and nearly all body processes.

The experiment: For 12 weeks, 10 thirsty college students each drank four, eight, or 12 eight-ounce glasses of water per day in four-day test cycles. Between the test cycles the students consumed the amount of water they normally would, as well as during one other week during the study period. The students' hydration status was measured at the end of each four-day water consumption cycle. They also answered questions about their general well being.

The results: Drinking four glasses of water caused the students' blood plasma volume to fall five percent below those who drank eight glasses of water. Four glasses of water also produced more highly concentrated urine. Still, these physical symptoms of dehydration are considered mild--a suboptimal hydration level. What surprised the researchers was the effect this mild dehydration had on the students' well being. Reuters reports that when students drank the least amount of water, they reported feeling less energetic and less focused than when they drank more water.

If eight glasses of water a day is good, is more than that even better? Probably not, although it won't hurt you. Those who drank 12 glasses had blood plasma volumes that were 10 percent higher, but their well being was not impacted positively or negatively.

The findings were presented at the annual Experimental Biology meeting in San Diego, California.