Water makes up 55-75% of the human body. Not drinking enough water affects nearly every body function: from organs to metabolism to mood.

Drinking Water Helps Your Body Work, Look, and Feel Better. Last year, Jennifer Aniston was famously quoted crediting her habit of drinking plenty water with her health, appearance, and keeping her from getting "cranky." While Jennifer Aniston is wrong on bottled water, she's totally right on water itself and there are scientific studies to back her up.

1. Water can make you feel more optimistic - No seriously. A study published in January 2012 in the Journal of Nutrition on mild dehydration showed that, while a loss of 1 to 2% hydration didn't affect mental acuity or function, slight dehydration made subjects perceive tasks as much harder than their well-hydrated counterparts. Dehydrated subjects were also more prone to headache, loss of focus, fatigue and a general bad mood.
2. Water improves mental performance and reaction time - In a finding useful for elite athletes and office workers alike, scientists determined that reaction time and mental function was reduced during exercise tests when subjects were only 1-2% dehydrated.
3. Water helps clear your complexion - Jennifer Aniston isn't the only person who thinks so, but there's a lot of confusion on this point. Many people assume drinking water will reduce dry skin. But as Katie Rodan, a dermatologist in the San Francisco Bay area and a coauthor of Write Your Skin a Prescription for Change explains to Real Simple magazine: “'humans aren’t like plants. Our skin doesn’t perk up when we consume water.’ . . . In fact, when you ingest it, 'water doesn’t go straight to the skin,' she says. 'It goes through the intestines, gets absorbed into your bloodstream, and is filtered by kidneys. Then it hydrates cells.'" The real benefit to the skin from proper hydration is that water helps the body flush toxins and waste.
4. Water helps the body flush toxins, waste - Every day your kidneys process about 200 quarts of blood to filter out about two quarts of toxins and metabolic wastes. Dehydration reduces blood volume and kidney efficiency. According to the CDC, your body also uses water to flush waste through perspiration and your digestive system.
5. Water makes you feel full, and help you lose weight - A study in 2010 showed that drinking two glasses of water before every meal not only boosted weight loss, but helped subjects keep the weight off.
6. Water keeps cells functioning efficiently, boosts metabolism for weight loss - In a double whammy of weight loss effectiveness, water not only helps you eat less, it helps your body more efficiently process what you do consume. Scientists have shown that it only takes a few minutes for water to improve metabolism.

So how much water should you drink? The Mayo Clinic reports that the old adage of 8 glasses a day isn't that far off for most people. Recently, the "drink when you're thirsty" recommendation has become prominent, but the Water for Health Alliance in the UK reports that thirst indicates that you are already mildly dehydrated, so act fast. But that 8-glass recommendation is for any fluid, so don't overdo it. Too much water can be as bad for your health as too little.