Caffeine

Caffeine is the most widely consumed psychoactive substance in the world, found in coffee, tea, yerba mate, chocolate, energy drinks, many sodas, and some medications. For most adults, moderate intake (up to about 400 mg/day, or roughly four 8-oz cups of brewed coffee) is generally considered safe — but sensitivity varies dramatically. Genetics influence how fast the liver clears caffeine, which is why one person can drink espresso after dinner and sleep fine while another is wired from a morning cup.

In the body, caffeine blocks adenosine receptors (the molecule that signals tiredness), raises cortisol and adrenaline, increases gut motility, and stimulates acid production in the stomach. These are the same mechanisms behind both its benefits — alertness, mood lift, exercise performance — and its common side effects: jitteriness, anxiety, reflux, loose stools, and sleep disruption.

Sleep and timing

Caffeine's half-life averages 5–6 hours but ranges from 2 to over 9 hours depending on genetics, smoking, pregnancy, and medications. That means a 2pm coffee can still have a quarter of its caffeine active at 10pm. People who sleep poorly should try cutting off caffeine by early afternoon for two weeks before deciding whether it's a trigger.

Tolerance, withdrawal, and reflux

Regular caffeine intake builds tolerance: the alerting effect fades, but adenosine receptors upregulate. Skipping a usual dose causes withdrawal headaches, fatigue, and irritability that peak around 24–48 hours. Caffeine also relaxes the lower esophageal sphincter, which can worsen reflux — and on an empty stomach, the acid bump alone can trigger nausea or stomach pain.