Moving the afternoon coffee to loose leaf, how much caffeine am I actually drinking?

Think in grams of leaf rather than cups, because leaf weight sets the ceiling. Caffeine is roughly three percent of dry tea by weight, and the published per gram figures put black tea somewhere in the low to high twenties of milligrams per gram and green tea substantially lower, around half that at the bottom end.

So your 4g session has a ceiling somewhere around ninety to a hundred milligrams if you extracted absolutely everything from a black tea, and you never do. Spread over six steeps in an hour that is far less punchy than the same total dumped into one mug, but the total is still the total. Swapping to green at the same leaf weight is the change that actually halves the number.

Please ignore anyone who tells you a five second rinse decaffeinates the leaf. That claim has been circulating in tea shops for decades and every measurement I have seen puts the amount removed by a short rinse at a small fraction, nothing like the majority. The rinse is for waking the leaf up and washing off dust, which are perfectly good reasons to do it. It is not a caffeine strategy.

I will gently disagree that the number is the thing to optimise. I moved my afternoon drink to hojicha, which is roasted and genuinely low in caffeine, and my sleep improved out of proportion to the milligram difference. Part of that is that a roasted tea does not make me want to drink six cups of it the way a bright sencha does. Behaviour beats arithmetic here. Aged white and heavily roasted oolongs work the same way for me.

Four grams of black is roughly a modest coffee. Four grams of green is roughly half of that. If the sleep problem is genuinely a problem rather than a preference, that is worth raising with a doctor rather than solving through leaf selection.

I got this wrong in the most obvious way available. Sat down at four in the afternoon with 6g of a young sheng, did nine infusions because it was good, and was awake until three. Six grams is a lot of leaf and I had somehow decided that because it was tea the arithmetic did not apply. Afternoon sessions are 3g maximum now, and after five I do not start a session I will not finish before six.

The variable nobody mentions is that caffeine comes out fast and early. Hotter water and longer steeps pull it quickly, so your first two infusions carry most of it. If you want the ritual without the load, do the first two steeps, tip them, and drink from three onward. You lose some flavour and you lose a lot of the caffeine.