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synergy · 1
Theanine is studied most not on its own but together with caffeine: in Owen 2008, 100 mg of theanine with 50 mg of caffeine made attention switching more accurate and people less easily distracted than placebo; but the trial had no theanine-only group, so it cannot say how much of the effect belongs to theanine. That it takes the edge off caffeine's jitters is a common marketing claim these trials did not measure.
antagonism · 4
↮Sleep Architecture & Sleep Debt
In the two-process model of sleep, Process S is sleep pressure: the longer you are awake, the higher it climbs, and the buildup of adenosine is one of its molecular signals. Caffeine works by blocking adenosine receptors; it does not remove the sleepiness, it just keeps you from feeling it for a while.
Caffeine and melatonin start out on two separate lines: caffeine blocks the adenosine line of sleep pressure, while melatonin is the body clock's signal that night has come. But evening caffeine also moves the clock itself later: in Burke 2015, caffeine equal to a double espresso taken 3 hours before bed delayed the melatonin rhythm by about 40 minutes, nearly half the delay from 3 hours of bright evening light. So a late-afternoon coffee plus a bedtime melatonin tablet has the first pushing back the very clock the second is meant to set.
↮Iron
What blocks non-heme iron is the polyphenols in tea and coffee, not caffeine itself: black tea drunk with a meal cut that meal's non-heme iron absorption by 79–94% (Hurrell 1999, against water with the same bread meal). People who need more iron can simply move tea and coffee to between meals.
A sleep hygiene checklist is not wrong; it removes obvious obstacles. Caffeine has a half-life of about 5–6 hours, with wide individual differences, so stopping early in the afternoon makes sense. But in most trials of cognitive behavioral therapy for insomnia, sleep hygiene is what the control group gets; on its own it does not cure chronic insomnia.
depletes · 1
Caffeine does slightly depress calcium absorption in the gut, but only very slightly, and it has no effect on the total calcium passed in urine over 24 hours (Heaney 2002). How slight? One or two tablespoons of milk fully offset it. In people who meet their calcium needs there is no evidence that caffeine harms bone. It is worth knowing precisely because it is small: a real interaction that marketing has inflated.
regulates · 5
Caffeine is shaped like adenosine: it occupies the adenosine receptors on neurons without switching them on, keeping the time-to-sleep signal locked out. It does not give you energy; it only mutes the fatigue signal for a while.
Caffeine is cleared slowly: it usually takes several hours for the amount in the blood to fall by half, and people differ widely. So a good part of an afternoon coffee is often still in your body when you lie down. It need not keep you awake to matter; even when you do fall asleep, the quality of deep sleep may suffer. So saying you sleep fine after coffee and saying the cup had no effect on you are two different statements.
Pairing caffeine with theanine may take some of the edge and jitteriness off the lift, but that only smooths the curve; it does not raise alertness. On a night without sleep, it improves how the hours feel, not how alert you can get.
Caffeine does more than keep you awake; it can also delay the body clock. So for shift workers, when you drink it matters more than how much: keep coffee to the first half of the shift so it does not push an already misaligned clock even further.
The link between coffee and palpitations is weaker than people assume. In one randomized trial, coffee days brought no clear rise in atrial premature beats, about half again as many ventricular ones, and 36 fewer minutes of sleep; another study of 1388 older adults found habitual coffee and tea unrelated to premature beats. If you suspect coffee, test it by alternating days rather than cutting it out wholesale.
contrast · 1
On the brain-supplement shelf, the combination with small-trial support is caffeine plus theanine: in a trial of 27 people, taking them together made attention switching faster and more accurate. Adding cholines on top has no such evidence. Before stacking, ask what you are trying to fix: attention held down by adenosine, or a shortage of acetylcholine?