A cup before the late-afternoon meeting, another before the evening work session. You are fully aware you run your day on coffee, and yet you file the trouble falling asleep at night as a separate matter — that was me, not so long ago. What connects the two is the idea of the "half-life": the time it takes for half the caffeine you drank to clear from your body. Learn this number, and the afternoon coffee starts to look different.
Clearing just half takes hours
According to a fact sheet compiled by the Food Safety Commission of Japan (Cabinet Office), the half-life of caffeine in the blood of healthy adults is roughly 2 to 8 hours. The range is wide because the speed of metabolism varies greatly with constitution, physical condition, and whatever else you are taking. Assume a half-life of 5 hours and run the numbers: a cup of coffee at 3 p.m. (call it just over 100 mg of caffeine) still has half remaining at 8 p.m., and a quarter still in your system at 1 a.m. The one working overtime past midnight wasn't you — it was your midday coffee.
Caffeine is thought to suppress sleepiness by temporarily blocking the action of a substance in the brain that signals drowsiness. In other words, the sleepiness doesn't disappear; the notifications are just muted. As long as caffeine remains in your system, the mute stays on even after you get into bed.
Two guideposts: "400 mg a day" and "nothing from evening on"
There are public reference points for quantity as well. The Food Safety Commission's fact sheet, drawing on assessments by the European Food Safety Authority and others, presents the evaluations of various bodies that intakes up to about 400 mg a day for healthy adults are considered not to increase health risks. In coffee, that is roughly three to four cups. From the sleep side, meanwhile, the Ministry of Health, Labour and Welfare's "Sleep Guide for Health Promotion 2023" touches on refraining from caffeine from the evening onward. The total by midday, zero from evening on — stack the two guideposts, and that is the operating rule you get.
What's interesting is that the large individual differences in caffeine tolerance are stated explicitly in the official documents. People who say "I can drink coffee at night and sleep just fine" really do exist. Still, the possibility remains that they fall asleep but sleep more shallowly. Self-reported tolerance can be unreliable.
The deadline is bedtime minus eight hours
- Set your own last call. If you sleep at midnight, that's 4 p.m.; if your nights still feel shallow, move it up to 2 p.m. Individual variation is large, so the only way to set your deadline is to experiment on yourself.
- From evening on, switch to decaf or barley tea. If what you're actually hooked on is the act of drinking coffee, swapping the contents is often all it takes.
- If you pair it with a midday nap, drink right before. The 20-30 minutes before the caffeine kicks in maps neatly onto the nap — it comes online just as you wake.
Some energy drinks and anti-drowsiness pills contain caffeine in a different league entirely. Check the label, and if you have health concerns or are pregnant, consult a doctor.
