HPC › Nutrition › Glossary
Caffeine is a methylxanthine alkaloid that competitively blocks adenosine receptors, specifically the A1 and A2A subtypes, in the central nervous system. This blockade prevents the inhibitory fatigue signals adenosine accumulates over waking hours. Absorbed rapidly from the gut and metabolised primarily by CYP1A2 in the liver, it carries a mean plasma half-life of approximately five hours.
It is the most widely consumed behaviourally active substance in the world; its effects arise from adenosine receptor blockade, not phosphodiesterase inhibition.
How it works
Caffeine exerts its wakefulness-promoting effects by competing with adenosine at A1 and A2A receptor subtypes across the central nervous system. 1 2 Adenosine, a metabolic byproduct of neural activity, accumulates throughout the day and, when bound to these receptors, progressively suppresses dopaminergic, noradrenergic, and cholinergic neurotransmission. Caffeine, structurally similar to adenosine, occupies the same binding sites without activating them. The fatigue signal is masked rather than resolved.
Metabolically, approximately 80-90% of a caffeine dose is N-3 demethylated to paraxanthine by the CYP1A2 enzyme in the liver. 5 The mean plasma half-life is approximately five hours, but the actual range spans 1.5 to 9.5 hours, driven by variation in CYP1A2 activity that is partly genetic. A fast metaboliser clears a 200 mg dose in around three hours; a slow metaboliser may still carry half of it well into the night.
Chronic daily consumption induces upregulation of adenosine A1 and A2A receptors: the brain responds to persistent blockade by increasing receptor density. 3 At higher receptor density, endogenous adenosine reasserts its inhibitory tone even at the usual caffeine dose. This tolerance develops within approximately four days of sustained intake at 3 mg/kg and reverses after a washout period of roughly ten days.
Example
An endurance athlete consumes 4 mg/kg approximately 60 minutes before a morning competition, within the window of peak plasma concentration. A colleague with the same habit drinks coffee late each afternoon to push through an energy dip. Same substance, same dose: the afternoon user's CYP1A2 genotype processes it slowly, and caffeine is still circulating at meaningful concentrations when sleep is attempted six hours later.
Timing and genetics together determine whether caffeine is a performance asset or a sleep liability.
Why it matters
Caffeine's most consistent cognitive benefit is on sustained attention and reaction time, with effects most robust under conditions of sleep deprivation. 4 The practical value is floor-raising rather than peak enhancement: caffeine compresses the gap between rested and sleep-deprived function. Shift workers, travellers managing jet lag, and anyone sustaining alertness under extended demand all benefit from this property, provided the dose is timed against natural adenosine accumulation windows and cleared before sleep.
Tolerance is the central constraint on strategic use. A consistent daily dose at 3 mg/kg degrades in efficacy within approximately four days; restoring full sensitivity requires roughly ten days without caffeine. 3 For athletes relying on acute ergogenic benefit, treating caffeine as a daily habit rather than a periodised tool effectively eliminates its competitive advantage. The performance difference between habituated and sensitised response is large enough to affect outcomes.
How does caffeine keep you awake?+
Caffeine blocks adenosine receptors in the brain, preventing adenosine from exerting its sleep-promoting and fatigue-inducing effects. Adenosine accumulates continuously during waking hours and, when it binds A1 and A2A receptors, signals the brain to slow down. Caffeine occupies those binding sites without activating them, masking the accumulated fatigue signal.
Why does caffeine stop working over time?+
Tolerance develops because the brain responds to prolonged adenosine receptor blockade by upregulating those receptors; more receptors means endogenous adenosine can reassert its inhibitory effects even at the usual dose. Sensitivity is recoverable: a washout period of approximately ten days restores receptor density and full responsiveness to caffeine's acute effects.
How much caffeine improves mental and physical performance?+
Effective ergogenic doses for both cognitive and physical performance range from 3 to 6 mg/kg of body weight, taken approximately 60 minutes before the target task. Individual response varies substantially with CYP1A2 genotype, which determines how quickly caffeine is cleared, and with habitual intake level, which determines tolerance.
How late in the day can you drink caffeine without disrupting sleep?+
Given a mean plasma half-life of five hours, avoid caffeine within eight to ten hours of intended sleep. Slow CYP1A2 metabolisers, for whom the half-life may approach 9.5 hours, should extend this window further. A dose consumed in mid-afternoon can still exert measurable stimulant effects late in the evening.
