Sarah Pullman has what she calls a long-standing, cyclical relationship with caffeine. When she doesn't have her morning latte, the John F. Kennedy University graduate student feels sleepy and her head pounds. When she has too much caffeine, she feels jittery. But most of the time, caffeine makes her feel perky.

``I'm able to focus and concentrate better,'' says Pullman, 30, of Oakland. ``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 2 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.

ENERGY DRINKS

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 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.

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