Since the two compete, the more caffeine you drink, the less adenosine is available up to a point. That's why caffeine temporarily heightens concentration and wards off fatigue. Within 30 to 60 minutes of drinking a cup of coffee, caffeine reaches peak concentrations in the bloodstream. It typically takes four to six hours for its effects to wear off.
Smokers remove caffeine from their blood twice as fast as nonsmokers.
That may be why smokers tend to drink more coffee. As little as 200 mg of caffeine is enough to make some people feel nervous and anxious. It might take even less for cola-guzzling kids. The typical American drinks about two cups of coffee a day. In 1962, when coffee consumption hit its peak, three cups was typical.