I drink a double espresso and forty-five minutes later I am face down on the sofa. This has been happening for years. Mrs Backhouse finds it funny. I find it puzzling enough that I finally looked it up, and the explanation turns out to be genuinely strange and worth knowing about.

The coffee does not make me sleepy. What makes me sleepy is the coffee stopping.

Here is how it works. Your brain produces a chemical called adenosine throughout the day. It accumulates as the hours pass, and the more of it there is, the stronger the signal to your body that sleep is due. Caffeine does not remove it. Caffeine sits in the adenosine receptors and blocks the signal, like putting a finger over the end of a hosepipe. The pressure builds behind it, and when the caffeine clears, everything that had been held back comes flooding through at once.

That rebound is where the sofa comes in. A double espresso is a large dose, so when it clears, the rebound hits hard. And the timing fits almost exactly. Caffeine peaks around thirty to forty-five minutes after you drink it. For fast metabolisers the half-life is roughly three to five hours, but the peak itself lands fast. Which means if the rebound catches you at the tail of that curve, the sofa arrives on schedule.

The gene part

There is a liver enzyme called CYP1A2 that handles around ninety-five percent of caffeine metabolism. If you carry two copies of the fast-metabolising version of that gene, caffeine clears your system quickly. About half the population does. Fast metabolisers are, in the words of one doctor quoted in the research, the people who can have an espresso and go right to sleep.

There is a second gene worth knowing about, called ADORA2A, which controls the sensitivity of your adenosine receptors – the exact targets that caffeine blocks. Variations there can amplify how hard the rebound lands when the caffeine clears.

Neither of these is a disorder. They are genetic variants. Roughly half of people metabolise caffeine fast. They are not weak or easily tired. They are processing the coffee quickly and then meeting the accumulated adenosine on the other side of it.

I have not done a DNA test to confirm any of this. But the consistency of the forty-five-minute window, across years and different coffees and different days, suggests fast metabolism is the most likely explanation. The body is not broken. It is following its own instructions.

I find this oddly reassuring. There is a logic to it that I did not have before. The sofa was not a failure of will. It was a hosepipe doing what hosepipes do when you take your finger away.

Mrs Backhouse still finds it funny. But now I have a reason.