It Helps. Caffeine combats drowsiness by tricking your brain into feeling alert. It temporarily blocks adenosine, a naturally sedating brain chemical, to prevent fatigue. "If you don't get a full night's sleep, you'll wake up with more adenosine in your brain than you normally would," explains Timothy Roehrs of the Sleep Disorders and Research Center at Henry Ford Hospital in Detroit. A hit of caffeine neutralizes adenosine and helps you feel less sleepy. If you're a regular coffee guzzler, though, you may need an extra boost to counter your late night. "As tolerance develops, the brain makes more receptors for adenosine," says Roehrs. "So you need more caffeine to block the added receptors."
It Helps. People who don't use caffeine regularly "usually become significantly more alert and better able to perform cognitive and motor tasks — like paying attention during boring or rote routines such as typing — if they're given the proper caffeine dose," says Laura Juliano, a professor of psychology at American University in Washington, DC. (For people who do use caffeine regularly, however, it offers few, if any, benefits.)
It Helps. "Caffeine can improve physical performance in an endurance exercise like running, but the effect is less for short bursts of movement such as lifting weights or sprinting," says Matthew Ganio, a professor of kinesiology at the University of Arkansas. Caffeine prompts the body to burn more fat stores instead of the limited stores of carbohydrate in our muscles. When the muscles run out of carbohydrate, you get tired. The benefit may be smaller in regular caffeine users.
It Helps. In a study of more than 300,000 U.S. men and women, those who consumed at least 600 mg of caffeine a day (an eight-ounce cup of coffee has between 95 and 200 mg) were about 30 percent less likely to be diagnosed with Parkinson's over the following ten years than those who consumed the least caffeine (less than 20 mg a day). Parkinson's patients gradually lose the nerve cells that produce the neurotransmitter dopamine. Caffeine protects those nerve cells.
It Helps. "In mice, caffeine not only defends against inevitable memory impairment," says Gary Arendash of the University of South Florida, "but also substantially decreases the amount of beta-amyloid, the bad protein that many researchers believe is the root cause of the disease." The few human studies have been inconsistent. In a Hawaiian study that tracked nearly 3,500 middle-aged men for 25 years, those who had reported consuming at least 400 mg of caffeine a day were 55 percent less likely to have brain lesions characteristic of dementia at their death than those who said they consumed less than 140 mg a day. However, they were no less likely to be diagnosed with dementia during their lifetime.
It Helps. When the pain comes on, the blood vessels in your brain widen; caffeine constricts them. It's also a mild pain reliever.
It May Hurt. The March of Dimes recommends that women who are pregnant or trying to become pregnant consume no more than 200 mg of caffeine a day because the harmful effects of more than that on fertility and fetal health "cannot be ruled out."
It May Hurt. People don't realize how much caffeine affects their sleep," says Juliano. "For those who are slow metabolizers of caffeine, there's still enough in their system to disrupt sleep at night even if they stop consuming it much earlier in the day." People who go off caffeine typically say they sleep longer and sounder, both Roehrs and Juliano report.
It Doesn't Matter. In a study that followed more than 130,000 men and women for 30 years, drinking coffee (regular or decaf) didn't increase the risk of cardiac arrhythmias, even among those with existing heart conditions.
It Doesn't Matter. Many companies add caffeine to weight-loss pills because it speeds up the metabolic rate, at least for a short period of time. Yet "there's little evidence that consuming caffeine leads to significant weight loss or helps people keep weight off," says Ganio.