Caffeine
A natural stimulant in coffee, tea, cocoa and some plants, acting on the central nervous system by blocking adenosine receptors (the tiredness molecule).
Caffeine is the most widely consumed psychostimulant in the world. Its mechanism: it blocks adenosine receptors. Adenosine is a molecule that builds up in the brain through the day and signals tiredness. By occupying these receptors, caffeine « masks » tiredness without removing it.
The original angle (adenosine debt and timing): a crucial and little-known point: caffeine only delays the perception of tiredness, it does not erase it. Adenosine keeps building up « behind » it; when the caffeine wears off, it is released all at once, causing the notorious « crash« . Hence the temptation to have another, a vicious circle.
The optimal timing:
- Not too early in the morning: cortisol is already high on waking (~6-9am), so caffeine adds little and you build up tolerance. Waiting ~90 minutes after waking would be more effective
- Not too late: caffeine has a half-life of 5-6 hours (sometimes longer). A coffee at 4pm still leaves ~50% of the caffeine active at 10pm, disrupting deep sleep, even in people who « sleep fine anyway » (their sleep quality is degraded without their knowing)
Genetics and tolerance: how fast you metabolise caffeine is genetic (the CYP1A2 gene). « Slow metabolisers » feel the effects (and the cardiovascular drawbacks) for longer; « fast » ones clear it quickly. This explains why some people sleep after an espresso and others don’t.
Documented benefits (at moderate doses): alertness, physical and cognitive performance, the antioxidant effect of coffee (rich in polyphenols), and an association with a reduced risk of certain diseases (Parkinson’s, type 2 diabetes) in observational studies.
Precautions: anxiety, palpitations, sleep problems, dependence (withdrawal causes headaches and fatigue). Pregnancy and certain conditions call for moderation.