"Coffee is a drug. Every drug has its ritual."
Caffeine mimics a neurotransmitter called adenosine which promotes sleep, inhibits arousal and regulates blood flow. The fact that caffeine makes you feel awake is not due to the caffeine but rather the ability caffeine has to mimic adenosine causing dopamine and glutamate (stimulants) to kick in - the real cause of your energy boost! Read more...
The reason people say coffee dehydrates you is that caffeine is a strong a diuretic. This is due to increasing the blood flow through the kidneys which is why coffee makes you urinate more than usual.
Caffeine has a small effect on respiration by increasing blood flow through the lungs and increasing the supply of air by relaxing bronchiolar and alveolar smooth muscle. That's why caffeine is effective in treating the breathing problems of some prematurely born infants!
Early experiments with caffeine showed that low concentrations of caffeine may produce small decreases in heart rate, whereas higher concentrations may make the heart beat abnormally fast.
Caffeine has a half-life of around 4 hours. While the half life of caffeine in the human body varies between 3 to 7 hours females metabolise caffeine 20-30% more quickly than males. However, it will take women on "the pill" twice as long to metabolise caffeine as women who are ovulating!
Brewed filter or "drip" coffee contains up to four times as much caffeine as espresso. While espresso seems stronger due to it's intensity a filter coffee is brewed for longer and generally contains far more caffeine due to portion size.
For more information on caffeine check out some of our sources.
Sources: http://www.abc.net.au/quantum/poison/caffeine/caffeine.htm, http://www.forbes.com/sites/daviddisalvo/2012/07/26/what-caffeine-really-does-to-your-brain, en.wikipedia.org/wiki/Adenosine, http://coffeechemistry.com/caffeine/caffeine-content-in-espresso-vs-drip-coffee1.html