Coffee consumption accounts for about 75% of the adult intake of caffeine, although my son maybe in part changing that with the growing popularity of energy drinks. However, out of all the energy drinks (and there are 286 at last count) there are only 10 that have above 51.3 mg/floz of caffeine which is the same as an ave 45ml shot of espresso. I know what I would choose. 

Caffeine is a bitter, white crystalline xanthine alkaloid that acts as a stimulant drug and a reversible acetylcholinesterase inhibitor. 

However - caffeine content in coffee varies greatly and depends on the beans, how they're roasted, and other factors, but the average for an 8-ounce cup is about 108 milligrams (mg). Tea has about half as much caffeine as coffee. Decaffeinated coffee has some caffeine, but the 2 to 4 mg in an 8-ounce cup is a smidgen compared with the caffeinated version. A lethal dose of caffeine is about 10 grams, which is equivalent to the amount of caffeine in around 100 cups of coffee. Caffeine gets absorbed in the stomach and small intestine and then distributed throughout the body, including the brain. The amount circulating in the blood peaks 30 to 45 minutes after it's ingested and only small amounts are around eight to 10 hours later. In between, the amount circulating declines as caffeine gets metabolized in the liver. Tobacco and marijuana accelerate caffeine metabolism, (although I not sure why you would want to smoke marijuana and drink coffee at the same time for efffect) forwhich reduces the time caffeine circulates in the body.

Interesting to note though that oral contraceptives slow it down, so they have the opposite effect. Research has identified genes that influence a person's natural risk of caffeine metabolism, which might explain why some people are exquisitely sensitive to caffeine while others are not. What is really interesting is that caffeine has multiple targets in the brain, but the main one seems to be adenosine receptors. Adenosine is a brain chemical that dampens brain activity. By hogging adenosine's receptors, caffeine sets off a chain of events that affects the activity of dopamine, another important brain chemical, and the areas of the brain involved in arousal, pleasure, and thinking. Also a part of the brain affected by Parkinson's disease, called the striatum, has many adenosine receptors; by docking on them, caffeine seems to have some protective effects. Outside the brain, caffeine can be a performance enhancer, boosting the strength of muscle contraction and offsetting some of the physiological and psychological effects of physical exertion.

Sleep tight.....