Coffee does not give you energy. It stops your brain from receiving one of the signals that says you are tired. That small difference explains why caffeine works, why it can let you down, and why a cup at four in the afternoon can still be costing you sleep at midnight.
What builds up while you are awake
Working neurons burn ATP, the cell's energy currency, and adenosine is one of its breakdown products (the energy bill is the subject of Your Brain Is an Energy Hog). In a classic experiment on cats, adenosine in the basal forebrain, a region involved in arousal, rose during six hours of enforced wakefulness and then fell during recovery sleep.1 That pattern made adenosine a leading candidate for one of the signals that ties sleepiness to how long you have been awake. It is not the only such signal, but it is the one caffeine interferes with.
What caffeine does
Caffeine is shaped enough like adenosine to fit into adenosine receptors on brain cells. It sits in the receptor without switching it on, and the real adenosine cannot get in. A comprehensive review concluded that at the doses people normally drink, blocking adenosine receptors is caffeine's main action in the brain.2
The receptor type matters. In mice, caffeine increased wakefulness in normal animals and in animals missing the A1 receptor, but not in animals missing the A2A receptor, which points to A2A as the one that carries caffeine's wake-promoting effect.3
Masking is not the same as resting
Caffeine blocks the signal. It does not clear the adenosine. When the caffeine wears off, the signal you were holding back can be felt again, which is a common explanation for the afternoon slump. The details in people are less clear than the popular version suggests, so treat it as a likely story, not a settled fact.
Why late coffee costs sleep
In a small laboratory study, twelve healthy adults took 400 mg of caffeine (roughly two to three cups of coffee) at bedtime, three hours before bed or six hours before bed, on separate occasions. All three timings disrupted sleep. With the dose taken six hours before bed, total sleep time measured by a monitor at home fell by more than an hour, and the volunteers did not report noticing it in their sleep diaries.4
That dose is large and the group was small, so do not read the numbers as a rule for your own cup. The useful lesson is the last part: you can lose sleep to caffeine without feeling that you did.
How much is reasonable
The European Food Safety Authority reviewed the evidence in 2015. It found that up to 400 mg a day, and single doses up to 200 mg, do not raise safety concerns for healthy adults. For pregnant and breastfeeding women it gave a lower limit of 200 mg a day. It also noted that doses as low as about 100 mg can affect sleep in some people when taken close to bedtime.5
People differ a great deal in how quickly they clear caffeine, so the average hides a wide range.
What to do with this
- Run a one-week test. Move your last caffeinated drink earlier in the day and notice how you sleep. It is one person's experiment, which is exactly why it is useful.
- Count every source. Tea, cola, energy drinks, chocolate and some pain-relief tablets all contain caffeine.
- Treat tiredness as information. If you need caffeine to function every day, the question is usually your sleep, which What Sleep Does for Your Brain covers.
General information. This article is not medical advice. If you are pregnant, have a heart rhythm condition or an anxiety disorder, or take medicines that interact with caffeine, ask your doctor what is right for you.
References
- Porkka-Heiskanen T, Strecker RE, Thakkar M, Bjørkum AA, Greene RW, McCarley RW. Adenosine: a mediator of the sleep-inducing effects of prolonged wakefulness. Science. 1997;276(5316):1265–1268. Find it ↗
- Fredholm BB, Bättig K, Holmén J, Nehlig A, Zvartau EE. Actions of caffeine in the brain with special reference to factors that contribute to its widespread use. Pharmacol Rev. 1999;51(1):83–133. Find it ↗
- Huang ZL, Qu WM, Eguchi N, et al. Adenosine A2A, but not A1, receptors mediate the arousal effect of caffeine. Nat Neurosci. 2005;8(7):858–859. Find it ↗
- Drake C, Roehrs T, Shambroom J, Roth T. Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed. J Clin Sleep Med. 2013;9(11):1195–1200. Find it ↗
- EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA). Scientific opinion on the safety of caffeine. EFSA J. 2015;13(5):4102. Find it ↗
Educational content, not medical advice. If you have a health concern, speak to a qualified professional.