Food component
Caffeine review — effects on alertness and attention, and daily limits
Also known as: coffee caffeine, caffeine and focus
At a glanceCaffeine masks the brain's sleepiness signal and reliably boosts alertness. Adults should stay under 400 mg a day, and late-day caffeine cuts sleep.
| Effect examined | Evidence | What studies found |
|---|---|---|
| Alertness and attention | Consistent evidence | At usual doses, studies consistently find better alertness, reaction time and sustained attention, with clearer effects when people are short on sleep. |
| Memory | Limited evidence | A 2014 study found that people who took 200 mg right after studying images were better at telling similar images apart a day later, but more confirmation is needed. |
| Sleep disruption | Consistent evidence | In an experiment, caffeine taken even six hours before bedtime reduced actual sleep time. |
How it works in the brain
While we are awake, a molecule called adenosine builds up in the brain, and when it binds its receptors we feel drowsy. Caffeine is shaped enough like adenosine to occupy those receptors instead and block the signal. Fatigue does not disappear; the “sleepy” signal is temporarily masked.
What studies show
Alertness and attention. Many studies have found that caffeine improves alertness, reaction time and the ability to sustain attention. A 2016 review concluded that these effects are especially clear when people are sleep-deprived.
Memory. In a 2014 study in Nature Neuroscience, people who took 200 mg of caffeine right after viewing pictures were better a day later at telling the originals from similar lookalikes. That suggests caffeine may influence how memories are consolidated, but it is a single study so far.
Sleep. In an experiment where people took 400 mg of caffeine zero, three or six hours before bed, even the six-hour dose cut actual sleep time by more than an hour. An afternoon coffee can affect the night’s sleep.
How much is too much
Korea’s Ministry of Food and Drug Safety recommends no more than 400 mg a day for adults, 300 mg for pregnant women and 2.5 mg per kilogram of body weight for children and adolescents. In its 2015 assessment, the European Food Safety Authority likewise found no safety concerns for healthy adults at up to 400 mg a day. The caffeine in a cup of coffee varies widely by product and brewing method, so the label is the most reliable guide.
Good to know
- Caffeine is also in tea, energy drinks, chocolate and some medicines, so add up everything consumed in a day.
- Limits for adolescents depend on body weight — 125 mg a day at 50 kg and 150 mg at 60 kg.
- Korea's official notice for L-theanine supplements advises care when combining them with caffeinated drinks.
This review does not recommend any product. If you take medication, are pregnant or breastfeeding, or have a health condition, talk to a doctor or pharmacist first.
References
- MFDS press release on maximum recommended caffeine intake (Korean). Ministry of Food and Drug Safety, 2026-10-20
- Scientific Opinion on the safety of caffeine. EFSA NDA Panel. EFSA Journal 13, 4102 (2015)
- Actions of caffeine in the brain with special reference to factors that contribute to its widespread use. Fredholm BB, Bättig K, Holmén J, Nehlig A, Zvartau EE. Pharmacological Reviews 51, 83–133 (1999)
- A review of caffeine's effects on cognitive, physical and occupational performance. McLellan TM, Caldwell JA, Lieberman HR. Neuroscience & Biobehavioral Reviews 71, 294–312 (2016)
- Post-study caffeine administration enhances memory consolidation in humans. Borota D et al. Nature Neuroscience 17, 201–203 (2014)
- Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed. Drake C, Roehrs T, Shambroom J, Roth T. Journal of Clinical Sleep Medicine 9, 1195–1200 (2013)