- "In healthy adults, caffeine's half-life is approximately 3–4 hours. In women taking oral contraceptives this is increased to 5–10 hours" [Wikipedia, original source] In other words, caffeine affects women on the Pill for twice as long as it does for most other adults, but it probably affects people not on the Pill more quickly and more strongly.
- Caffeine on its own is a stimulant in the body. Caffeine metabolizes in the liver into three different other chemicals - most becomes paraxanthine (84%, takes fat out of storage and into the bloodstream, raising blood sugar levels, and sugar is a stimulant), theobromine (12%, diuretic and a stimulant, is present in chocolate and is why most people mistakenly think that chocolate has lots of caffeine), and theophylline (4%, a good asthma treatment, but in much higher concentrations than produced by caffeine metabolism, also raises blood pressure). Each of these by products appears to be created by removing a CH_3 from caffeine and replacing it with a Hydrogen, the only question is which one gets removed; each of them has further stimulant effects. [Wikipedia: Caffeine, Theobromine]
- Chocolate contains both caffeine and theobromine, but significantly more theobromine, and only about as much caffeine as a cup of decaf coffee. Theobromine is often confused with caffeine however, and the media does not distinguish between the two, so the misconception that chocolate is a significant source of caffeine continues to perpetuate.
- One of the ways caffeine works is that it "competes" with adenosine, so that it essentially stops adenosine from working. Adenosine is a depressant in the human nervous system, and its presence is related to the need for sleep; its lack means that dopamine instead begins to dominate - and dopamine is a stimulant, increasing heart rate and blood pressure.
- In humans, theobromine has less of an effect on the nervous system than does caffeine, but has more of an effect on the heart. In addition, theobromine relaxes smooth muscles (ones that act involuntarily), such as dilating blood vessels (and thereby decreasing blood pressure) and bronchial tubes in the lungs (possibly explaining why I've always felt that caffeinated beverages help when I'm feeling allergic, as I also have allergy-triggered asthma).
- For reasons unclear to me, most animals (including dogs, cats, and birds) process theobromine more slowly than humans, with the result that it affects them more strongly, leading to theobromine poisoning. Do NOT feed your pets any of your leftover holiday chocolates. It could be the last thing they eat.
Well, that's a bunch of disorganized facts for you. :-P I warn you of two things though, (1)
ETA: Another link here to birds and theobromine.