While we know that not having enough to drink in extreme heat, or when vigorously exercising makes people very unhappy (and quite ill), it's less clear how small changes in fluid intake affect mood and energy levels.

A new study , led by Coleen Muoz, and to be published in Appetite recruited 120 healthy female college students who documented everything they ate and drank for five days, and filled well-respected mood questionnaires. The researchers calculated the water intake from all sources (food and drink), and controlled for known factors that affect mood, such as exercise and caffeine. All the participants were on oral contraceptives, so that fluctuations in hormonal levels would be less of a factor.

What they found was that the greater the water consumption, the better the mood. Tension, depression and confusion scores went down when water intake went up.

After reading this study two questions come to my mind.

First: Do upbeat women drink more water, or does water make women more upbeat?

This study doesn't answer this question. As mentioned before, there really isn't a rich amount of data on this topic, but in another study , published last year in PLoS ONE, 30 people who habitually drank about five cups a day were asked to increase their intake to about 10 cups a day for three days. Mood improved, as well as energy levels and satisfaction. To further prove this point the researchers assigned 22 people who regularly drank plenty of water to decrease their intake. And the result: low water intake led to worse moods, more headaches, confusion and tiredness.

And the second question: Who funded this study? Well, there is a conflict of interest here; Danone Research (Danone is Evian 's parent company) financed this study, as they do many others related to hydration -- including that other study in PLoS ONE I mentioned above. Which is not to say that I don't believe the results and that the results were in any way manipulated. Some foods and nutrients are backed by research money, and many times when healthy foods like vegetables, fruit, olive oil, spices and water are studied we're encouraged to hear that our grandmas had a good hunch, and that these also perform well in studies. That's also not to say that orphan plant foods that don't have an industry sponsor are any less beneficial.

Water for happiness? Perfectly worth a try, it comes right out of your tap and it's calorie free.

A few tips about water intake as warm weather heads our way:

How much: The eight glasses a day isn't based by any solid evidence -- so no need to count -- but is pretty good approximation for an average person in average circumstances. The Institute of Medicine (IOM) doesn't set exact needs , but recommends for women about 2.7 liters (91 ounces) of total water a day, and for men about 3.7 liters (125 ounces) daily -- and no upper limit for water. Eight glasses is about 64 ounces, so if you add to that the water in food, this isn't far off from the IOM recommendations. Water needs depend, of course, on activity level, outdoor temperature etc.

What counts: All fluids count, including coffee, milk, soup and the plenty-of water in your fruits and veggies (watermelon is 90 percent water).

How to you know if you're well hydrated: One should rarely feel thirsty, and your urine should be almost colorless.

Err on the side of more: Under normal circumstances, in healthy people, the kidneys regulate fluid levels. Drinking too little leads to headaches, dehydration, constipation and maybe crankiness, while it's almost impossible to drink too much. Only in extreme circumstances -- endurance athletes heavily exerting while overdrinking water - can drinking too much water become a problem.

Dr. Ayala

Full disclosure: I'm vice president of product development for Herbal Water, where we make organic herb-infused waters that have zero calories and no sugar or artificial ingredients. I'm also a pediatrician and have been promoting good nutrition and healthy lifestyle for many years.