Smithsonian’s Surprising Science:
Soon after you drink (or eat) something containing caffeine, it’s absorbed through the small intestine and dissolved into the bloodstream. Because the chemical is both water- and fat-soluble (meaning that it can dissolve in water-based solutions—think blood—as well as fat-based substances, such as our cell membranes), it’s able to penetrate the blood-brain barrier and enter the brain.
Structurally, caffeine closely resembles a molecule that’s naturally present in our brain, called adenosine (which is a byproduct of many cellular processes, including cellular respiration)—so much so, in fact, that caffeine can fit neatly into our brain cells’ receptors for adenosine, effectively blocking them off. Normally, the adenosine produced over time locks into these receptors and produces a feeling of tiredness.
Caffeine is physically addictive. Your brain actually starts to increase the number of adenosine receptors in response to overstimulation. If you drink coffee or tea on a regular basis, that “boost” you get with your morning cup of coffee is probably just the relief of withdrawal symptoms. To find out what your normal baseline is, you need about 1.5 to 2 weeks to reset.
Caffeine is most beneficial when taken occasionally, in moderate doses. The effective amount varies, depending on sex (women process caffeine faster than men), habits (smokers eliminate caffeine 30 to 50% faster), body size, and individual metabolism. Between 50 mg on the low end, and about 300 mg on the high end is optimal. More than 500 mg in a day can cause negative effects and more serious withdrawal symptoms.
A Starbucks Grande brewed coffee (16 fl oz) has about 330 mg of caffeine, which is an average individual’s limit for the day. Lattes contain about 150 mg, so you can have about two before encountering some negative symptoms.
If you have insomnia, better lay off the caffeine entirely, or at least limit consumption to before noon. The half-life in adult men is between 2.5 and 4.5 hours, so sleep patterns can be affected long after the last dose.