Caffeine is not a stimulant in the way a loud alarm is a stimulant. It works by blocking adenosine, the molecule that accumulates while you are awake and makes you feel sleepy. It does not remove the sleepiness; it hides it. When the caffeine wears off, the adenosine is still there — often in a larger pile than before.
The arithmetic
The half-life of caffeine in a typical adult is around five hours, with a normal range of roughly three to seven. That means half of what you drank is still circulating five hours later, a quarter after ten hours, and so on.
A rough example: a 200 mg coffee at 3 p.m. leaves about 100 mg at 8 p.m., 50 mg at 1 a.m. and 25 mg at 6 a.m. That is not nothing. For a person who is sensitive to caffeine, 50 mg at 1 a.m. is enough to lighten sleep and increase awakenings without any obvious feeling of being “wired.”
Why your mileage varies
- Genetics. Variations in the CYP1A2 enzyme change how fast caffeine is cleared.
- Medications and hormones. Some contraceptives and other drugs slow clearance substantially.
- Smoking. It speeds clearance — which is not a reason to start.
- Pregnancy. Clearance slows, and the recommended limit is lower.
- Habit. Regular use changes how many adenosine receptors are available, which is why the same cup can feel different over time.
If you are awake at 3 a.m. and cannot find a reason, look at what you drank at 3 p.m.
What to do
A practical starting point is a cutoff eight to ten hours before bed: no coffee, strong tea, cola or energy drinks after early afternoon. If you are sensitive, try twelve hours for two weeks and see what the sleep diary says.
Decaf is not zero caffeine, and chocolate, some teas and many soft drinks count. The point is not purity; it is knowing what you are actually asking your brain to ignore.