It almost sounds like the benefits of some promising new blockbuster drug: Just a little bit can help lift mood, concentration and energy levels -- with virtually no side effects.

Well, there is one side effect: You might need to pee a little more.

Of course, this isn't some experimental new drug -- just plain old water, and new research shows how letting yourself run dry can throw your whole day out of whack.

In a set of three experiments, 25 healthy young women were given either enough H20 to keep hydrated or brought just below their optimal levels with exercise and diuretics.

By "just below" I mean really just below -- they were missing only between 1 and 2 percent of their needed water. But those small changes led to big differences as these women suffered from measurable dips in mood and focus and were more likely to feel fatigue and suffer headaches.

Although the study didn't look at men, there's no reason to think mild dehydration would affect them any differently.

In other words, you need your water -- but don't get carried away.

You know the old saying about drinking eight 8-ounce glasses a day? Forget it. It's never been proven by science (although the bottled water companies would like you to think otherwise).

In fact, too much water can be even worse for you than too little.

The only time you need to wet your whistle is when you feel thirsty -- and despite what you've heard, it doesn't have to be plain old water.

Coffee and tea, for example, are just fine. Both of them are mildly diuretic, but they'll still leave you with a net gain of water and keep you hydrated.

Just one warning here: Water may not be a drug on its own... but there's a good chance there are drugs in your water.

U.S. water standards are plunging like a barrel over Niagara Falls. Hormones, sex meds, antibiotics and more are regularly turning up in our drinking water -- and in some places, you can add illegal drugs, rocket fuel and toxic waste to the list.

Even in trace amounts, do you really want to drink that?

Drink only filtered water. You don't have to shell out big money on bottled year after year if you invest in a quality reverse osmosis system for your home.