Why Your 3pm Coffee Keeps You Up: What Your Caffeine Genes Can Tell You
•Posted on October 22 2026
Two people have the same flat white at three in the afternoon. One sleeps like a baby. The other is staring at the ceiling at midnight. Same coffee, different genes.
I talk through this on a solo episode of the Pushing the Limits podcast. You can listen, or read the full transcript, on the episode page. Here is the caffeine and sleep side of it.
Why "no coffee after lunch" doesn't work for everyone
Most of us have been handed blanket advice: two coffees max, nothing after lunch. But blanket advice assumes we are all the same, and we are not. Your genes influence how fast you clear caffeine and how strongly your brain responds to it.
CYP1A2: how long caffeine hangs around
More than 90 percent of the caffeine you drink is broken down in your liver by one enzyme, made from the CYP1A2 gene. There is a well-studied variant in that gene linked to how quickly the enzyme can work. Genetic tests, including the myDNA test I use in my practice, group people as fast, intermediate or slow metabolisers. Fast metabolisers tend to clear caffeine more quickly. Intermediate and slow metabolisers tend to hang on to it for longer.
Your genes give you a strong clue, but not a stopwatch. This variant mostly changes how strongly the enzyme ramps up when it is pushed, for example in heavy coffee drinkers and smokers. How quickly you actually clear caffeine also depends on things like smoking, medications and hormones.
Even fast metabolisers have caffeine on board at bedtime
For most adults it takes several hours just to clear half of a dose, so a good chunk of a 3pm coffee can still be on board at bedtime. And you can't always feel it. Studies have found that caffeine can cut into your sleep even when you don't notice any difference in how you slept.[1] So when someone tells me caffeine doesn't affect them, I always want to know how they are actually sleeping.
Finding your cut-off time
If you come back as an intermediate or slow metaboliser, the advice is not never drink coffee again. It is to move your cut-off earlier. Enjoy the coffee in the morning and drop the afternoon one. If you clear caffeine slowly you can still enjoy coffee. You just have it at 8 in the morning, not 4 in the afternoon.
This enzyme is not fixed, either. In one study, a month of hard training ramped it up substantially, and eating plenty of broccoli every day nudged it up a little.[2] Genes and environment work together, but timing is the simplest lever by far.
For athletes: caffeine before every session?
Caffeine is one of the most researched performance aids in sport, and I used it occasionally in my ultramarathons. But when researchers split people by this gene, the response was not always the same.[3] In one of the biggest studies, over 100 athletes did a 10 kilometre cycling time trial.[3] The fast metabolisers rode faster on caffeine, the intermediate group saw no real change, and the small slow group rode slower on the higher dose.[4]
Across the studies the pattern leans the same way, but it is not a universal rule and the research is still developing.[5] If you are in a group that gets little benefit, you could be having caffeine before every session, getting nothing extra from it and paying for it with your sleep that night. Sleep is when so much of your recovery happens.
ADORA2A: how strongly you feel it
There is a second gene in the panel worth knowing about, ADORA2A. It makes one of the receptors in your brain that caffeine blocks, and that blocking is a big part of how caffeine keeps you alert. Research links variants in this gene to how prone you are to the jittery, anxious side of caffeine, and in some studies to how much caffeine disturbs your sleep.[6]
CYP1A2 gives clues about how long caffeine hangs around. ADORA2A helps explain how strongly you feel it. Neither is the whole story, but together they tell you a lot.
Find out where you sit
Genes load the gun and environment pulls the trigger. Your genes show you where your weak spots are, so you know where to focus. If you are wondering which group you are in, that is what the DNA Blueprint Package answers. You get the full myDNA comprehensive health report covering 92 genes and 113 variants across eight areas of your health, your own written personal health program, and a 90 minute one-on-one with me to go through it all. Because your genes don't change, you only need to do it once, and I think it is the most valuable test you could ever do. It is available in New Zealand, Australia and the USA. Find out more at lisatamati.com/dna.
Educational content only. Genetics shows tendencies and priorities, not diagnoses.
References
- Naturalistic Effects of Five Days of Bedtime Caffeine Use on Sleep, Next-Day Cognitive Performance, and Mood. (2014), PubMed 24868491
- The Effects of Cruciferous Vegetable-Enriched Diets on Drug Metabolism: A Systematic Review and Meta-Analysis of Dietary Intervention Trials in Humans. (2020), PubMed 32086800
- Caffeine, CYP1A2 Genotype, and Endurance Performance in Athletes. (2018), PubMed 29509641
- Effect of Caffeine on Endurance Performance in Athletes May Depend on HTR2A and CYP1A2 Genotypes. (2022), PubMed 32569126
- CYP1A2 genotype and acute ergogenic effects of caffeine intake on exercise performance: a systematic review. (2021), PubMed 33137206
- Genetics of caffeine and brain-related outcomes - a systematic review of observational studies and randomized trials. (2023), PubMed 37029915
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