Caffeine has a half-life of about five hours in most healthy adults, which means half of a dose is still in you five hours after you took it. A 100mg dose at 3pm leaves roughly 50mg at 8pm and 25mg at 1am.
Half-life is the useful number because it explains the thing people actually notice: caffeine does not stop, it fades. The feeling goes long before the caffeine does.
Five hours is an average and the spread is wide. Smoking roughly halves it. Pregnancy and some medications can double it. If caffeine seems to affect you far more or far less than the people around you, that is normal and it is largely genetic.
A mug of filter coffee is around 95mg, instant is around 60mg, and a single espresso is around 63mg. A large coffee-shop drink can reach 200mg or more, because it contains two or three shots.
The range is the point. Nobody hands you a coffee with the milligrams on it, and two flat whites from two different cafes can differ by a factor of three. That is the argument for a measured format — not that it is better for you, but that you can count it.
For most healthy adults, caffeine at normal intakes is not considered harmful. EFSA's review put habitual intake up to 400mg a day, and single doses up to 200mg, as raising no safety concern. Above that, the common effects are a faster heartbeat, anxiety and disrupted sleep.
It is a stimulant, not a health product, and it does not replace sleep. It defers tiredness — the sleep debt is still there when it wears off.
Lower limits apply in pregnancy and while breastfeeding, and caffeine is not recommended for children. Anyone with a heart condition, an anxiety disorder, or medication that interacts with caffeine should ask a pharmacist or GP first.
A caffeine crash is the tiredness you were already carrying arriving all at once as the caffeine fades. Caffeine blocks the signal that tells you you are tired; it does not remove the tiredness. A smaller dose softens the drop, but only sleep fixes it.
This is the main practical argument for 50mg over 150mg. A smaller dose fades more gently, because there is less of it to wear off.
Not meaningfully at normal intakes. Caffeine is a mild diuretic, but the fluid in a caffeinated drink more than offsets what it makes you lose, and regular consumers build tolerance to the effect. Caffeine taken without a drink has no fluid to offset and no meaningful dehydrating effect either.
Work back from your bedtime using caffeine's roughly five-hour half-life. If you sleep at 11pm, take a 100mg dose before 1pm and a 50mg dose before about 6pm. Later than that and a measurable amount is still circulating when you are trying to sleep.
Falling asleep is not the only thing at stake. Caffeine late in the day reduces deep sleep even when you do drop off, which is why people who get eight hours after an evening coffee can still wake up tired.