Caffeine, Done Right: Dose, Timing and the Afternoon Cut-Off

Caffeine is the most widely used psychoactive substance in the world, with decades of consistent evidence behind it. A large review found reliable benefits for vigilance, attention and reaction time across a wide range of doses, especially when people are tired (McLellan et al., 2016).

Most people still use it badly: too much, too late, with no plan. Here’s how to get the upside without the crash.

How it works

While you’re awake, a molecule called adenosine builds up in your brain and makes you feel progressively sleepier. Caffeine sits in adenosine’s receptors without activating them, so you don’t feel the build-up.

Two consequences:

1. Caffeine doesn’t create energy. It hides tiredness. The adenosine is still there.
2. When caffeine wears off, that backlog arrives at once. That’s the crash.

The numbers that matter

Drink	Typical caffeine
Mug of filter coffee	~140 mg
Mug of instant coffee	~100 mg
Single espresso	~60 to 75 mg
Mug of tea	~75 mg
250 ml energy drink	~80 mg
Can of cola	~40 mg

Typical UK figures from NHS and Food Standards Agency guidance. Coffee-shop drinks can be much stronger.

Five rules

1. Less is usually more

Benefits plateau quickly; side effects don’t. For most people, 100 to 200 mg per session is the sweet spot.

2. Respect the afternoon cut-off

With a ~5-hour half-life, half of a 3 pm coffee is still circulating at 8 pm. In a sleep-lab study, 400 mg taken six hours before bed cut total sleep by more than an hour (Drake et al., 2013). A good default is last caffeine 8 to 10 hours before bed. More in Sleep Is the Stack.

3. Pair it with L-theanine

If caffeine makes you jittery, this is the best-evidenced fix. L-theanine blunted caffeine’s blood-pressure rise and improved attention in combination trials (full breakdown).

4. Watch the tolerance trap

Daily users develop tolerance. Some researchers argue that much of what heavy users feel from their morning coffee is relief from overnight withdrawal rather than a true boost (James & Rogers, 2005). Keep your daily dose moderate and steady, and save bigger doses for days that need them.

5. Try a coffee nap

Drink a coffee, then immediately nap for 15 to 20 minutes. You wake up just as the caffeine kicks in. In a driving-simulator study, caffeine plus a short nap reduced sleepiness more than either alone (Reyner & Horne, 1997).

Myths to drop

• “Wait 90 minutes after waking for your first coffee.” Popular online, but there’s little direct evidence it improves focus or prevents a crash. Do it if it suits you; don’t suffer through grogginess for it.
• “Caffeine dehydrates you.” At normal intakes, caffeinated drinks count towards your fluids.
• “More caffeine, more focus.” See rule 1.

Who should go easy

• Pregnant or breastfeeding: 200 mg a day maximum.
• Anxiety or panic disorder, heart rhythm problems, high blood pressure: speak to a GP first.
• Under 18s: much lower limits apply.

References

General information, not medical advice. Supplements and stimulants affect people differently, so check with a GP or pharmacist before starting anything new, especially if you're pregnant, breastfeeding, on medication or have a health condition.