Found this here: http://www.howstuffworks.com/caffeine.htm
Caffeine is known medically as trimethylxanthine, and the chemical formula is C8H10N4O2. When isolated in pure form, caffeine is a white crystalline powder that tastes very bitter. The chief source of pure caffeine is the process of decaffeinating coffee and tea.
Medically, caffeine is useful as a cardiac stimulant and also as a mild diuretic (it increases urine production). Recreationally, it is used to provide a “boost of energy” or a feeling of heightened alertness. It’s often used to stay awake longer - college students and drivers use it to stay awake late into the night. Many people feel as though they “cannot function” in the morning without a cup of coffee to provide caffeine and the boost it gives them.
Caffeine is an addictive drug. Among its many actions, it operates using the same mechanisms that amphetamines, cocaine and heroin use to stimulate the brain. On a spectrum, caffeine’s effects are more mild than amphetamines, cocaine and heroin, but it is manipulating the same channels and that is one of the things that gives caffeine its addictive qualities. If you feel like you cannot function without it and must consume it every day, then you are addicted to caffeine.
And here: http://www.water.com/your_health_news/hs_gh_art002_p01.asp
Proper hydration is critical during heat waves when loss of water due to extreme heat and humidity can be potentially life-threatening. Water has an essential role in maintaining health. These facts summarize how the heat affects the body.
Heat and the Body:
Extreme heat and humidity rob the body of water through perspiration and respiration, resulting in the following:
The body’s natural balance is disrupted, since water is responsible for dispersing nutrients throughout the body while expelling toxins.
The body loses its natural energy, causing fatigue and sluggishness.
The body loses vital electrolytes like sodium, potassium, and chlorides. These electrolytes are critical to your body.
On a normal day, the body loses about two quarts of water. Doctors and nutritionists recommend that a person drink between one-half and two-thirds of an ounce of water daily for every pound of body weight. The more active you are, the closer to the two-thirds of an ounce you need. This means about 9 to 10 eight-ounce servings of water per day for an active person.
During heat waves, people experience more water loss.You should drink at least one additional quart of water per day to compensate for this loss.
Everyone spending time outdoors should drink water—before, during and after sun exposure.
So basically, the reason that I believe that so many people are tired all the time is that they just don’t drink enough water. Also, IIRC, if you pee clear, you good to go, if it’s yellow you need to drink more water.