Your 4 PM coffee is still in your system at midnight

In 2013, Christopher Drake and colleagues ran the experiment every coffee lover dreads. Twelve healthy sleepers, at home, with objective sleep monitors. On different nights they took 400 mg of caffeine — roughly two strong coffees — at bedtime, 3 hours before bed, or 6 hours before bed. Other nights: placebo. Nobody knew which was which.

The result

Even the 6-hour group lost more than an hour of total sleep compared to placebo nights. And here's the cruel part: most participants didn't subjectively feel the disruption. They thought they slept fine. The monitors disagreed.

The math

Caffeine's half-life is about 5–6 hours. That 4 PM espresso (200 mg) is still ~100 mg at 10 PM and ~50 mg at 4 AM — right in the middle of your deep sleep. Your brain is trying to file the day's memories while a stimulant keeps tapping it on the shoulder.