Caffeine: blocking the pressure signal
You will be able to: Explain how caffeine works on sleep pressure, and why the NHS advises a 6-hour gap before bed.
How caffeine works
Remember adenosine, the chemical that helps make you feel sleepy? To have its effect, adenosine has to fit into special landing sites on brain cells, called receptors.
Caffeine blocks those receptors. While caffeine is sitting in them, adenosine's "time to sleep" message gets through more weakly, and you feel more alert.
That explains two things you may have noticed:
- It does not repay sleep debt. You feel less sleepy, but caffeine does not replace a good night's sleep. Module 1's debt is still on the books.
- It can spoil the night after. Even if you fall asleep after caffeine, you may not sleep as deeply. A review of the research found that caffeine typically made it take longer to fall asleep, shortened sleep and reduced deep sleep.
Why it lasts so long
Your body clears caffeine slowly. In healthy adults, it takes about 4 hours on average to clear half of it, but that ranges from 2 to 8 hours between people. Age, body weight, pregnancy, some medicines and liver health all make a difference.
So a coffee at 16:00 is not "gone" by bedtime. At the average rate, about a quarter of it would still be in your body at midnight. For a slow clearer, about half.
How much is in a drink?
The amount varies a lot between brands and cup sizes, but the NHS gives these estimates:
| Drink | Typical caffeine |
|---|---|
| Cup of instant coffee | 100 mg |
| Cup of tea | 75 mg |
| Can of energy drink (250 ml) | 80 mg |
| Can of cola | 40 mg |
| 50 g bar of plain dark chocolate | less than 25 mg |
A large coffee-shop coffee can contain more than a cup of instant coffee (our estimate for a 12 fl oz cup is about 150 mg). Caffeine is also found in some tablets and some health or fitness supplements: check the label.
Myth or fact? Tap each card
Caffeine gives you energy.
It works mainly by blocking the action of adenosine, a chemical that helps make you feel sleepy. It does not replace sleep.
Source: NINDS, Brain basics: understanding sleep; Sleep Foundation, Caffeine and sleep.Tea has hardly any caffeine.
The NHS estimates about 75 mg in a cup of tea, not far off a cup of instant coffee at about 100 mg.
Source: NHS, Foods to avoid in pregnancy.A coffee at 4 pm is long gone by bedtime.
It takes about 4 hours on average, and up to 8, to clear just half of it.
Source: EFSA (2015).Energy drinks always have far more caffeine than coffee.
The NHS estimates about 80 mg in a 250 ml can, a little less than a cup of instant coffee. Bigger cans can hold more, so check the label.
Source: NHS, Foods to avoid in pregnancy.Quick check (3 questions)
Sources
- NINDS, Brain basics: understanding sleep (adenosine helps make you feel sleepy; caffeine blocks its actions)
- Sleep Foundation (medically reviewed), Caffeine and sleep (blocks adenosine receptors; does not replace a good night's rest; may not sleep as deeply)
- Clark & Landolt (2017), Coffee, caffeine, and sleep: a systematic review, Sleep Medicine Reviews 31: 70-78, PubMed 26899133
- EFSA (2015), Caffeine explainer (PDF)
- Drake et al. (2013), J Clin Sleep Med 9(11): 1195-200, PubMed 24235903
- NHS, Insomnia; NHS, Foods to avoid in pregnancy (caffeine per drink; 200 mg a day in pregnancy)
- NHS, Heart palpitations (caffeine as a common cause; when to see a GP, call 111 or call 999); NHS, Tiredness and fatigue (see a GP if you are tired all the time and do not know why)
- FDA, Spilling the beans: how much caffeine is too much? (basis of our 150 mg estimate)
Next: your caffeine cut-off calculator