The rate at which caffeine is metabolized, however, can vary depending on body weight, genetic luck of the draw, lifestyle, and even what you had for lunch.
Women on birth control pills, for example, take longer to metabolize caffeine, and so get more mileage out of a cup of coffee or a bottle of caffeinated soda; smokers, on the other hand, metabolize caffeine twice as fast as normal, and so get less of a buzz.
On the other hand, not everyone reacts to caffeine in the same way.
On average, the half-life of caffeine in the human body is about five hours – that is, it takes five hours to flush out 50 percent of the caffeine in your system.
Caffeine doesn’t disrupt REM (Rapid Eye Movement) sleep like alcohol does; instead it appears to interfere with stages three and four sleep, the periods when we get our most restful and restorative down time.