How it works and how much
As you stay awake, adenosine builds up and makes you sleepy; caffeine blocks its receptors so you feel less tired. The FDA says up to 400 mg a day is generally safe for healthy adults, and ACOG advises staying under 200 mg in pregnancy. A 16-oz coffee shop brew can top 300 mg, and energy drinks range widely, so it's easy to overshoot.
Sleep, side effects, and health
With a half-life of about five hours, an afternoon coffee can still be half active at bedtime, worsening insomnia and shifting your circadian rhythm. Too much causes jitters, anxiety, a racing heart, and acid reflux; quitting abruptly brings headaches. A 2017 BMJ umbrella review linked 3–4 cups of coffee a day to lower risk of several diseases, but those are observational findings, and pregnancy is an exception.
This article is general health information and is not a substitute for diagnosis or treatment. If symptoms persist, see a doctor.
FAQ
- Does decaf have caffeine?
- A little, usually about 2–15 mg per cup.
- Is the caffeine in tea gentler?
- Tea has less caffeine, and L-theanine may make the boost feel smoother.
Sources reviewed: 2026-10-26
References
- U.S. Food and Drug Administration. (2024). Spilling the Beans: How Much Caffeine Is Too Much?
- European Food Safety Authority. (2015). Scientific opinion on the safety of caffeine. EFSA Journal, 13(5), 4102.
- Poole, R., et al. (2017). Coffee consumption and health: Umbrella review of meta-analyses of multiple health outcomes. BMJ, 359, j5024.