Caffeine

What caffeine does in your body

Caffeine is a naturally occurring alkaloid found in over 60 plant species, including coffee beans, tea leaves, and cacao. Chemically known as 1,3,7-trimethylxanthine, it's the most widely consumed psychoactive substance on the planet. About 85% of American adults consume it daily, averaging around 135 mg per day.

Once ingested, caffeine reaches peak blood levels within 30-60 minutes. It works primarily by blocking adenosine receptors in the brain. Adenosine is a molecule that accumulates during waking hours and gradually makes you feel sleepy. Caffeine fits into those same receptors without activating them, effectively masking the "time to sleep" signal ^[1]. This is why coffee makes you feel alert rather than actually restoring energy.

Beyond adenosine, caffeine also increases dopamine and norepinephrine signaling, which improves mood, reaction time, and sustained attention. It stimulates the release of adrenaline, raises heart rate slightly, and boosts metabolic rate by 3-11% in the short term.

Caffeine and your genes: fast vs. slow metabolizers

How your body handles caffeine depends heavily on a single gene: CYP1A2. This gene encodes the liver enzyme responsible for metabolizing over 95% of ingested caffeine. A common polymorphism (rs762551) divides people into two camps ^[2]:

• Fast metabolizers (AA genotype) break down caffeine roughly four times faster than slow metabolizers
• Slow metabolizers (AC or CC genotype) experience prolonged effects, more sleep disruption, and higher cardiovascular sensitivity to caffeine

This genetic variation explains why one person can drink espresso after dinner and sleep fine, while another feels wired from a single morning cup. If caffeine consistently disrupts your sleep or causes anxiety, you may be a slow metabolizer. Consumer genetic tests now include CYP1A2 status, and the information can genuinely improve how you time and dose caffeine.

Performance and cognitive effects

Caffeine is one of the most studied ergogenic aids in sports science. The International Society of Sports Nutrition confirms that doses of 3-6 mg per kg of body weight improve endurance performance, reaction time, muscular strength, and sprint ability ^[3]. For a 70 kg person, that's roughly 200-400 mg, or about 2-4 cups of coffee.

Interestingly, very low doses (around 100 mg, or one cup of coffee) already produce measurable cognitive benefits: improved attention, faster processing speed, and reduced mental fatigue ^[4]. Higher doses don't necessarily add more cognitive benefit but do increase side effects like jitteriness and anxiety.

Athletes often use caffeine strategically before competition. The key finding from recent meta-analyses: caffeine's ergogenic effect is strongest in people who don't consume it daily, because habitual intake blunts the performance boost through tolerance ^[3].

Caffeine tolerance and how to reset it

Tolerance develops within 3-7 days of regular use. Your brain responds to chronic adenosine receptor blockade by producing more adenosine receptors, which means you need more caffeine to achieve the same effect. This also creates physical dependence: withdrawal symptoms (headaches, fatigue, irritability) typically peak 24-48 hours after your last dose and resolve within a week.

A full tolerance reset takes 7-14 days of abstinence for most people. Practical approaches include:

• Cold turkey for 10-14 days (fastest but roughest)
• Gradual taper: reduce daily intake by 25% every 3-4 days
• Cycling: alternate weeks on and off to maintain sensitivity
• Weekly caffeine-free day to partially slow tolerance buildup

Caffeine, sleep, and timing

Caffeine's average half-life is 5-7 hours, meaning half the caffeine from a 2 PM coffee is still circulating at 8 PM. A 2023 systematic review confirmed that caffeine consumption increases sleep onset latency by an average of 9 minutes and reduces total sleep time, with the strongest effects on deep sleep (N3 stage) ^[1]. Even people who claim caffeine "doesn't affect my sleep" show measurable reductions in sleep quality on EEG recordings.

The practical rule: set a personal cutoff time at least 8-10 hours before bed. For slow metabolizers, that window should be even longer. Morning consumption (after the first 90 minutes of waking, when cortisol naturally peaks) tends to produce the best results without sleep disruption.

Caffeine and longevity

Large-scale epidemiological data consistently links moderate caffeine intake to reduced mortality. A meta-analysis of 40 studies covering 3.8 million participants found that 3-4 cups of coffee per day was associated with a 15% lower risk of all-cause mortality ^[5]. This benefit held across ages, BMI categories, smoking status, and even caffeine metabolizer genotype.

Caffeine also shows neuroprotective properties. Epidemiological studies report that regular caffeine consumers have a 30-60% lower risk of Parkinson's disease, with a dose-response relationship ^[6]. The evidence for Alzheimer's protection is suggestive but less conclusive: caffeine may reduce amyloid-beta accumulation and tau hyperphosphorylation through adenosine A2A receptor modulation ^[6].

Other longevity-relevant effects include reduced risk of type 2 diabetes, improved liver function, and activation of AMPK, a cellular energy sensor that plays a role in autophagy and metabolic regulation ^[7].

Safe intake and practical guidelines

The FDA, EFSA, and most health authorities agree on 400 mg per day as the safe upper limit for healthy adults. That's roughly 4-5 cups of brewed coffee. Pregnant women should limit intake to 200 mg per day.

• Start with 100-200 mg if you're caffeine-naive and assess tolerance
• Time your intake after the morning cortisol peak (90+ minutes after waking)
• Cut off caffeine 8-10 hours before sleep
• Consider cycling if you notice diminishing effects
• Avoid energy drinks with excessive caffeine combined with other stimulants
• If you experience anxiety, heart palpitations, or insomnia, reduce dose or get CYP1A2 tested