Why does coffee make you feel less sleepy?
The stars of this story are caffeine and adenosine.Because the caffeine in coffee blocks the receiving spots for adenosine, a substance that carries the sleepiness signal. Adenosine builds up in the body while you are awake and tells the brain, "Time to rest." Caffeine gets in the way of that signal, so you feel less sleepy.
The cause of sleepiness: adenosine
While we are awake and active, our bodies make a substance called adenosine and slowly build it up. According to the U.S. National Institutes of Health (NIH), the sleepiness you feel in the evening is a sign that adenosine has built up. Adenosine sends a signal that calms nerve activity by attaching to receiving spots on brain cells called receptors.
Caffeine blocks the receiving spots
Caffeine has a shape very similar to adenosine. Materials from Japan's Ministry of Agriculture, Forestry and Fisheries also explain that caffeine attaches to adenosine's receiving spots first, so adenosine can no longer attach. It is like a game of musical chairs where caffeine sits down in the chair first. As a result, the "time to rest" signal has a hard time getting through, and you feel less sleepy.
Picture caffeine sitting down first in a game of musical chairs.
Sleepiness doesn't go away
However, caffeine does not remove the cause of sleepiness. It only gets in the way of the signal. According to the NIH, if you take caffeine every day, your body gets used to it, and you may need more caffeine to feel the same wake-up effect.
The effect differs from person to person
How caffeine works differs from person to person. The U.S. Food and Drug Administration (FDA) explains that sensitivity to caffeine, and how quickly it leaves the body, vary greatly between people. Japan's Ministry of Agriculture, Forestry and Fisheries says that for children, people who are pregnant or breastfeeding, and people sensitive to caffeine, choosing products with the caffeine removed is one option. If you are concerned, it is best to talk to a doctor or another expert.
Want to know more?
- The FDA says that for most adults, 400 milligrams of caffeine a day is an amount not usually linked to harmful effects.
- Even decaf coffee, with the caffeine removed, still has a little caffeine left (2–15 milligrams per about 240 mL, according to the FDA).
- Materials from Japan's Ministry of Agriculture, Forestry and Fisheries say brewed coffee contains about 60 milligrams of caffeine per 100 mL.
Quick quiz: test yourself
How does caffeine make you less sleepy?
Show the answer
- It feeds the brain energy
- It raises body temperature
- It blocks the sleepiness signal
It blocks the sleepiness signal Caffeine blocks the receivers for adenosine, the substance that tells the brain to rest, so the signal can't get through.
Caffeine blocks the receivers for adenosine, the substance that tells the brain to rest, so the signal can't get through.
Next quiz ›Sources
- Tired or Wired? Caffeine and Your BrainNIH News in Health（アメリカ国立衛生研究所）
- カフェインの過剰摂取について農林水産省 (in Japanese)
- Spilling the Beans: How Much Caffeine is Too Much?U.S. Food and Drug Administration (FDA)
This article was written after checking the sources above as of 2026-10-03. Explanations may change as research progresses. Photo credits are shown below each photo.
