How many coffees a day? The number, and the cut-off.

How many cups of coffee a day do you actually get through? Count them: one when you get up, a flat white on the way in, one at eleven between meetings, one after lunch, one at three to get you to the evening.

If that makes five, you're far from alone. 'I love my coffee,' many of the people I work with tell me, and I hardly ever take it away. I look at two things: how many, and what time the last one lands.

How many cups of coffee a day is too many?

For a healthy adult, up to three a day. Four at the very most. A shot of espresso has about 100mg of caffeine, and as a general guide the ceiling is about 400mg a day. From around 300mg, some people start sleeping worse or notice their heart racing, so I settle on three.

• Tea. A cup of black tea has about 70mg, so a few mugs soon add up. Green tea has roughly 8 to 50mg.
• Dark chocolate. About 5mg a square.
• The blend. Robusta has roughly twice the caffeine of arabica.
• Doubles and fizzy drinks. A two-shot flat white counts as two. Cola counts too.

Why the fifth one doesn't wake you up

It hides the warning light without filling the tank. Through the day, a molecule that signals tiredness builds up in your brain. Caffeine takes its seat, so the signal can't get through. The tiredness is still there, even if you stop feeling it.

You get used to it. Drink it daily and you build tolerance: to get the lift of the first cup you'd need more and more. The fifth tends to bring jitters rather than focus.

It lingers for hours. Your body usually takes five to seven hours to clear half. A 3pm coffee is still partly in your system at nine. Sleep can suffer, and next morning you're likely to want one more cup.

Everyone clears it at their own speed. How fast your liver deals with caffeine depends largely on how you're built. If a mid-afternoon coffee leaves you staring at the ceiling, you probably clear it slowly, and those hours can stretch to eight or ten.

When to drink it, and how

First one with breakfast. In the morning the hormone that gets you going is already high, and coffee works with it. Have it after a glass of water. It peaks in about forty minutes.

Last one straight after lunch. That way there's less left by bedtime. If you miss the ritual mid-afternoon, make it a decaf.

Coffee on an empty stomach? Plenty of people have no trouble with it. But coffee makes you produce more stomach acid, so if it gives you heartburn first thing, have it after food.

Before a workout. A coffee thirty to sixty minutes beforehand can make the effort feel easier. It won't create new strength; it helps you use what you have. Before weights, 'I just had a coffee' falls short, so eat something. If you train after work, skip the coffee: the after-lunch rule still applies.

Over four? Come down by one. Cut the latest first and hold that number for a week. The discomfort usually passes in a few days.

Start with the timing, the easy part: for one week, last coffee straight after lunch. Then watch how you sleep.