Quick Answer: Caffeine lasts much longer than it feels like it does (a half-life of roughly 4 to 7 hours), it can reduce deep sleep even when you fall asleep easily, decaf still contains a little, and a single espresso usually has less caffeine than a mug of drip coffee. Here are the myths worth dropping.
1. "Caffeine wears off in a couple of hours"#
The buzz fades, but the caffeine doesn't. Its half-life is roughly 4 to 7 hours depending on the person, so half of a 3 PM coffee can still be in your system at 8 or 9 PM. The caffeine half-life explainer walks through the math.
2. "If I can fall asleep after coffee, it doesn't affect me"#
Falling asleep isn't the whole story. In a well-known study, 400 mg of caffeine taken even 6 hours before bed cut total sleep by about an hour, measured objectively (Drake et al., 2013, Journal of Clinical Sleep Medicine). Caffeine can also reduce deep sleep in people who drift off without trouble.
3. "Decaf has no caffeine"#
Decaf still contains a small amount, usually a few milligrams per cup, and it varies by brand and brewing. For most people it's not enough to matter, but several cups late at night add up if you're sensitive.
4. "Espresso is the strongest coffee"#
Per ounce, yes. Per serving, usually not: a single shot of espresso has roughly 60 to 65 mg of caffeine, while a standard mug of drip coffee has around 95 mg, and a large coffee-shop brew can have well over 200.
5. "Tea has more caffeine than coffee"#
Tea leaves contain more caffeine by dry weight than coffee beans, but you use far less of them. A brewed cup of black tea usually has around 40 to 50 mg, about half a typical cup of coffee.
6. "Coffee dehydrates you"#
Caffeine is a mild diuretic, but in moderate amounts coffee still counts toward your fluid intake. A study of regular coffee drinkers found four cups a day hydrated them about as well as the same amount of water (Killer et al., 2014, PLoS One).
7. "Coffee sobers you up"#
Caffeine can make you feel more alert, but it doesn't lower the alcohol in your blood or restore reaction time. Feeling awake and being sober aren't the same thing.
8. "You have to wait 90 minutes after waking for your first coffee"#
It's a popular recommendation, but the evidence is mixed. Some people find a later first cup softens the afternoon crash; others notice no difference. The best test is your own afternoons, one change at a time. Why delay your first coffee? covers both sides.
9. "Another coffee fixes the 3 PM crash"#
It masks it. Caffeine blocks the sleep-pressure signal without removing it, and a late cup can cost you sleep that night, which makes tomorrow's crash worse. A short walk outside, or a 20-minute nap, breaks the cycle instead.
There's no universal cutoff. For one average cup and an average metaboliser, caffeine takes roughly five hours to fall below 50 mg. A bigger cup, a slower metabolism, or an earlier coffee still in your system pushes that earlier. The free caffeine cutoff calculator works out yours, counting what you've already had today.
Where ARC Fits#
ARC tracks every coffee you log in one tap and recalculates your last safe coffee each time, from your bedtime and how fast you clear caffeine. When no cup fits before bed any more, it tells you plainly.