Evening Coffee Lingers Because the Liver Has to Clear It Twice

Anyone who has ordered coffee at 5 in the afternoon and then stared at the ceiling at 10 at night has run into a problem of liver plumbing. The body does not clear caffeine in one pass. It clears it in two slow steps, and the middle chemical counts, too.

First, a liver protein called CYP1A2, an enzyme that breaks down foreign chemicals, converts caffeine into a different stimulant called paraxanthine. That conversion alone takes about five hours to cut the caffeine in half.

Then the same enzyme gets to work on the paraxanthine, turning it into other chemicals that do not keep you awake. That second cleanup takes about three hours to cut the paraxanthine in half.

In one simplified model of caffeine metabolism, caffeine and paraxanthine are assumed to be equally stimulating, so as caffeine falls, paraxanthine rises to take its place. Only about 80 percent of caffeine becomes paraxanthine, with the rest going to less active byproducts, so the total stimulation does drift down. In this model, the effective half-life is close to 10 hours, which explains why an afternoon cup can still matter at bedtime.

One proposed idea from that simple model is to start with paraxanthine itself, leaving only the three-hour second step. A consumer maker called Enfinity sells paraxanthine and calls it "cleaner," with "fewer jitters and a better headspace." The writer of the analysis argues it should be safe because 80 percent of the caffeine people already drink turns into it in the body.

The early human reports are mixed. One early tester, a commenter called kman, reported that afternoon doses did not prevent sleep at 10 or 11 at night, and that a 7 p.m. dose still allowed sleep at 11:30. But another commenter, Tomas B, said it did basically nothing even at 200 milligrams. In a tally of 24 Reddit posts, about two thirds were positive and one third were negative, with some negative reviewers saying it put them to sleep.