"I'm able to focus and concentrate better," says Pullman, 30, of Oakland, Calif. "Sometimes, I think it enhances my mood, like 'Wow, I feel great. Yippee!'"

Turns out caffeine, a naturally occurring, flavorless chemical that stimulates the central nervous system, does more than wake us up. Medical researchers and nutritionists long have touted the health benefits of black coffee, antioxidant-packed teas and dark chocolate. In moderation, these natural sources of caffeine are associated with weight loss, the treatment of asthma and headaches and a reduced risk of Parkinson's disease. Coffee, in particular, is associated with a decreased risk of depression, colon cancer and type two diabetes, according to researchers at Vanderbilt University's Institute for Coffee Studies.

But that doesn't justify guzzling Joe all day. According to the American Medical Association, moderation is considered three cups of coffee a day, or 300 milligrams of caffeine. If you're a tea drinker, you can double that to six cups, because eight ounces of brewed tea has 30 to 50 milligrams of caffeine, says Junaid Khan, cardiac surgeon with Alta Bates Summit Medical Center in Berkeley, Calif. Too much dark chocolate obviously can lead to obesity, he adds, and so can sodas and other caffeinated products loaded with sugar. He cautions women who are pregnant, people with osteoporosis and diabetics to curb their caffeine use.

But Khan's only major concern over caffeine consumption is among young athletes who use energy drinks such as Rockstar, Red Bull or Spike Shooter. The latter has 300 milligrams — one's daily intake of caffeine — in a single serving. He sees it often: A young runner consumes two energy drinks before a marathon, and the combination brings on heart palpitations, sometimes to the point of causing the runner to pass out.

"The problem I have with those (drinks) is that people don't realize how much caffeine they're getting," he says. "The combination of sugar and caffeine can act as a diuretic, causing urination when the person is already dehydrated. Young kids are coming in with heart arrhythmias. They think (these energy drinks) are like Diet Coke, and they're not."

Vanessa Barahona tried taking an energy drink before a run once, but she didn't like the effect at all. "It felt like acid was running through my body," says Barahona, 21, of Lafayette, Calif. "I felt dehydrated. It was like the opposite of Gatorade." She sticks to mixing Red Bull with vodka at parties. She likes the taste, she says, and the added benefit of up being able to stay up late.

Caffeine is a stimulant, but the stimulation isn't just about speed. It's also about mental performance. "So it's not that you're working faster or have an edge, you actually perform better because of the caffeine," Khan says. He also cites a 2001 study that showed a 30 percent lower risk of Parkinson's disease in folks who drank three cups of coffee a day. It was the caffeine, he says, that was responsible for the reduction. "It is thought that caffeine may prevent the loss of dopamine, the critical brain chemical that is depleted by the disease," he explains.

Parkinson's isn't the only serious disease that may be prevented by taking caffeine. Peter Martin, a Vanderbilt University professor of psychiatry and pharmacology, says that one to three cups of coffee in particular has been proven to reduce the risk of type two diabetes by several percentage points.

"Here, it's not the caffeine but a plethora of compounds within coffee, including chlorogenic acids, that can reduce the risk not only of type two diabetes but also Alzheimer's disease and colon cancer," says Martin, who runs the university's Institute for Coffee Studies. "What we still need to understand is the mechanism by which it does this."

Martin also cites a Brazilian study that found the incidence of teen depression was lower in kids who grew up drinking coffee. "My hypothesis is if kids began drinking coffee before they began drinking alcohol and smoking pot and cigarettes, it may help in the prevention of severe addictions," he says.

Claudia Long, a Lafayette, Calif., attorney who grew up in Mexico, has no experience with depression, but she does say that caffeine makes her "feel better all around." Long, 54, has been drinking coffee since the age of 8. She calls herself the queen of caffeine.

"All my life I struggled with asthma, and just about the only thing that stopped an attack was a cup of that strong, dark stuff," she says. When she became pregnant, Long was forced to eliminate caffeine from her diet, and the attacks returned, she says. It wasn't until a doctor prescribed Theophylline, a medication that is structurally similar to caffeine and relaxes the bronchials in the same way, that her asthma stabilized.

"As soon as my son was born, my husband brought me a thermos of iced coffee in the recovery room," Long says.

Ria Tanz Kubota is a retired registered nurse who self-medicated for more than a decade using caffeine. As an adult with undiagnosed attention deficit hyperactivity disorder, she drank 10 to 12 cups a day to stay focused, she says.

"I could get jittery on cup nine or so, but I slept fine and was able to safely do my nursing work in the hospital," says Tanz Kubota, 67, of El Sobrante, Calif. During this time she noticed another positive side effect of caffeine. "A cup or two in the morning helps regularity. When I'd withdraw from it (caffeine), I'd go constipated immediately."

Caffeine does effect contraction in the colon, and while there's no data supporting coffee enemas and weight loss, Khan, the cardiac surgeon, says it's fine for use as a mild diuretic.

"If it keeps you regular, and you're within the AMA's standards for caffeine, go for it," Khan says. "Listen to your body. That's the most important thing you can do when it comes to caffeine."