Caffeine is the most widely used psychoactive substance in the world and most people misunderstand the mechanism, which explains several things about how it behaves.
What it does
Through the day, a molecule called adenosine accumulates in the brain and produces the sensation of tiredness. Caffeine has a similar shape and occupies the same receptors without activating them.
So it does not add energy. It blocks the perception of tiredness while the adenosine continues accumulating behind it.
Why the crash happens
When the caffeine clears, the accumulated adenosine reaches the now-free receptors at once. The tiredness arrives all together rather than gradually.
Why tolerance develops
The brain responds to blocked receptors by making more of them. After regular use, more caffeine is needed for the same effect, and without it there is now an excess of receptors for adenosine to bind – which is the withdrawal headache.
The half-life
Around five hours in most adults, meaning half is still present five hours later and a quarter after ten. A late-afternoon coffee is measurably present at bedtime.
This varies genetically by a wide margin. Some people clear it in two hours and some in nine, which is why advice about cut-off times cannot be universal.
What this suggests practically
Caffeine works best used deliberately rather than constantly, because tolerance blunts a habitual dose.
And if sleep is a problem, moving the last cup earlier is one of the more effective changes available.
The caveat
General information. Anyone with heart conditions, anxiety disorders, or in pregnancy should follow medical guidance on intake.