Coffee

Caffeine has a similar shape and takes its place. The signal is covered. But adenosine is still there, waiting.

Caffeine has a shape similar enough to fit the same receptors without activating them: it occupies the lock and adenosine cannot get in. The signal is covered, not switched off: adenosine keeps building up behind the door. When caffeine fades, it arrives all at once and sleepiness comes back stronger. People who drink coffee every day make more receptors, and the same cup does less: that is tolerance (Nehlig 2018; EFSA 2015).

Nehlig 2018 · DOI 10.1124/pr.117.014407

After a half-hour nap, 66 per cent. After a decaf, 13.

In the same study, after a 30-minute nap in the car 66% drove as they would by day; after a decaf (15 mg), 13%. Coffee and the nap both work, and the following night's sleep does not suffer. Limits stated by the authors: a single dose, a single nap length, only rested young men (Philip 2006).

Philip 2006 · DOI 10.7326/0003-4819-144-11-200606060-00004

Coffee doesn't clear quickly: every five hours, half of it is still there.

Caffeine does not end, it halves. The half-life in a healthy adult is about 5 hours, but ranges from 1.5 to 9.5 depending on the person (Institute of Medicine 2001): genes, smoking (shortens it), the contraceptive pill (doubles it), pregnancy. 95% is cleared by the liver through the enzyme CYP1A2 (Nehlig 2018). After 5 hours half is left, after 10 a quarter, after 15 an eighth.

Institute of Medicine 2001 · DOI 10.17226/10219

The nine o'clock cup is down to an eighth by midnight. The five o'clock cup, to more than a third.

With a 5-hour half-life, the 9 o'clock coffee is down to an eighth by midnight: almost gone. The 5 o'clock one, seven hours later, is still above a third (about 38%). And it disturbs sleep: 400 mg taken six hours before bed removes more than an hour of measured sleep, without the sleeper noticing (Drake 2013). Hence the widespread feeling of sleeping well even after an evening coffee: sleep gets worse, but it is not felt. Add the tolerance of daily coffee drinkers and, for a minority carrying the fast CYP1A2 variant, clearance within a few hours (Nehlig 2018).

Drake 2013 · DOI 10.5664/jcsm.3170

It isn't just coffee. Tea, cola, energy drinks, chocolate, painkillers: the total goes to bed with you.

Caffeine per serving (EFSA 2015): filter coffee 200 ml 90 mg, espresso 80, energy drink 250 ml 80, tea 220 ml 50, cola 355 ml 40, dark chocolate 50 g 25. Some painkillers contain it too. For an adult, up to 200 mg at once and 400 a day raise no concern (EFSA 2015): an espresso at the café, a cola at lunch, an energy drink at 5 and a painkiller in the evening go past that without anyone noticing. Carlos Schenck, one of the Monte Verità voices, talks about exactly this: caffeinated drinks in the evening and overstimulation that reaches the road.

EFSA 2015 · DOI 10.2903/j.efsa.2015.4102