Medical Today

Ninety percent of Americans consume caffeine every day in one form or another. Most people drink coffee, tea or other caffeinated beverages as pick-me-ups to increase alertness, or consume them as weight-loss supplements to lose body fat. Caffeine is a central nervous system stimulant that improves wakefulness, alertness and some types of athletic performance. It affects some people more than others, depending on tolerance, body size and gender.

A University of Barcelona (Spain) study showed that caffeine had a greater, faster effect in men than women. The scientists measured the short-term (first 30 minutes) and long-term (rest of the day) effects of caffeine ingestion in nearly 700 college students. Coffee increased alertness within 45 minutes, but the effects were greater in men than women. Decaffeinated coffee also increased alertness, although the effects were less than regular coffee.

In most people, the effects of caffeine last for 2-3 hours, but can last as long as 10-12 hours in pregnant women and in people with liver disease.