From Developing Intelligence:
Caffeine is the most widely used stimulant in the world, but few use it to maximal advantage. Get optimally wired with these tips.
1) Consume in small, frequent amounts.
Between 20-200mg per hour may be an optimal dose for cognitive function.
Caffeine crosses the blood-brain barrier quickly (owing to its lipid solubility) although it can take up to 45 minutes for full ingestion through the gastro-intestinal tract. Under normal conditions, this remains stable for around 1 hour before gradually clearing in the following 3-4 hours (depending on a variety of factors).
A landmark 2004 study showed that small hourly doses of caffeine (.3mg per kg of body weight [approx 20 mg per hour; thanks digg!]) can support extended wakefulness, potentially by counteracting the homeostatic sleep pressure, which builds slowly across the day and acts preferentially on the prefrontal cortex (an area of the brain thought responsible for executive and "higher" cognitive functions).
At doses of 600mg, caffeine's effects on cognitive performance are often comparable to those of modafinil, a best-of-class nootropic.
2) Play to your cognitive strengths while wired....MOREHT: the NYT's Freakonomics blog