The half-life is the thing nobody accounts for
Caffeine's half-life is roughly five hours, and varies a lot between people. A coffee at 4pm means a meaningful fraction is still circulating at 9pm and some of it at 2am. You will fall asleep — caffeine doesn't necessarily stop that — but your sleep quality is degraded, and sleep is where the day's studying gets consolidated.
So the 4pm coffee that got you through the afternoon quietly deleted part of the afternoon's learning. That's a poor trade and almost nobody notices they're making it.
Using it deliberately
- Before the hard session, not continuously. Caffeine sipped all day builds tolerance and produces no peak.
- Not after early afternoon, if you want your sleep. Adjust for your own sensitivity, which varies enormously.
- Keep a normal intake low so that a dose actually does something when you need it. Tolerance is the enemy of usefulness.
- Careful on exam morning. Too much converts alertness into jitter and anxiety, and anxiety costs you marks in the first twenty minutes. If you don't normally drink it, exam day is a spectacularly bad day to start.
What it doesn't do
It doesn't improve learning. It improves *alertness*, which lets you study when you'd otherwise be too tired — a real benefit, and a completely different claim. It does not make the studying itself more effective, and it does not substitute for sleep. It masks the deficit while the deficit compounds.