That 5 p.m. coffee costs you an hour of sleep

400 milligrams of caffeine, about four cups, was the dose Christopher Drake and his team at the Henry Ford Hospital in Detroit gave twelve healthy volunteers in a trial published in 2013. They took it at three different moments: right at bedtime, three hours before sleeping, and six hours before. Even the dose taken six hours ahead cut more than an hour off their sleep, measured with a sensor rather than by feel.

The reason is chemistry. Caffeine takes between four and six hours to fall to half its level inside the body: a 100-milligram coffee at five in the afternoon is still circulating, at half strength, when you turn off the light, and at three in the morning a quarter of it is still in there.

And here comes the uncomfortable part of the experiment: in their diaries, the volunteers wrote that they had slept well. Caffeine spoils your sleep and it also spoils the instrument you judge it with, which is how you feel the next morning. That is why the afternoon coffee seems harmless for years.

Count eight hours back from the time you go to bed and put your border there: if you fall asleep at midnight, the last coffee of the day is at four.