Studies disagree — unsettled
A professional body says genetic variation may explain why the same coffee helps one person and keeps another awake. It stops short of telling anyone to test.
The International Society of Sports Nutrition's 2021 position stand says that differences between individuals — in performance, and in the sleep and anxiety effects — may be attributed to genetic variation in caffeine metabolism and response. It also names habitual intake as a factor, and it does not recommend genotyping.
CYP1A2 is the enzyme that clears most of a dose. Studies of rs762551 and athletic performance genuinely disagree with each other, and the honest summary is that this is unsettled rather than established.
Which leaves the obvious test, available to everyone and free: drink a coffee at four in the afternoon and see whether you sleep.
Nothing on this page recommends taking anything. If a result here matters to you, take it to a doctor or a pharmacist rather than to a shop.