You and a friend both order a coffee at 3pm. She's asleep by 11. You're still wide awake at 1am. Habit and tolerance play a part, but some of that difference comes from a single gene that controls how quickly your body clears caffeine.
Meet CYP1A2
Most of the caffeine you drink is broken down in your liver by an enzyme called CYP1A2, and the gene that makes it varies from person to person. The best-studied variation sits at a spot labelled rs762551, where you can carry either an A or a C. Because you inherit one copy from each parent, everyone falls into one of three groups: AA, AC or CC.
If you're AA, your liver works through caffeine quickly, so an afternoon latte has largely worn off by bedtime. If you carry a C, the same drink stays in your system for longer, which is why a coffee you barely remember having at 3pm can still be keeping you awake at midnight.
What caffeine does to your sleep
In 2023, researchers pulled together the results of 24 studies on caffeine and sleep. When caffeine was in the system, people slept about 45 minutes less a night and lost around 11 minutes of deep sleep. Their sleep was less efficient, it took them longer to drift off, and they spent more time awake during the night.
The same review worked out how far ahead of bed caffeine needs to stop. For a cup of coffee, with about 107mg of caffeine, the answer was at least 8.8 hours. For a typical pre-workout scoop, with about 217mg, it was 13.2 hours. Black tea had no cut-off at all, because its effect on sleep wasn't large enough to show up.
Source: Gardiner et al., Sleep Medicine Reviews, 2023.
For someone going to bed at 11pm, that puts the last coffee of the day at about 2pm. Those numbers are averages across everyone, though, and your genotype moves your own cut-off a couple of hours earlier or later.
Your coffee day, by type
Based on an 11pm bedtime and a 7am alarm, here's how we'd time caffeine for each genotype.
| Genotype | Speed | Last coffee | Daily amount |
|---|---|---|---|
| AA | Fast | 4pm | Up to 400mg, no more than 200mg at a time |
| AC | Slower | 12pm | About 1 cup (around 100mg) |
| CC | Slow | 12pm | About 1 cup (around 100mg) |
If the 3pm slump hits and you carry a C, make it a black tea. If you train after work, skip the caffeinated pre-workout: a scoop needs 13 hours before bed, so for an 11pm bedtime the cut-off is about 10am.
What's actually in your cup
Caffeine advice is always given in milligrams, but nobody orders a coffee that way. Here's what common drinks contain.
| Drink | Caffeine |
|---|---|
| Starbucks Blonde Roast, grande | 360mg |
| Starbucks Pike Place, grande | 310mg |
| Celsius, 12oz can | 200mg |
| Monster, 16oz can | 160mg |
| Brewed coffee, 8oz cup | 96mg |
| Red Bull, 8.4oz can | 80mg |
| Black tea, 8oz cup | 48mg |
| Green tea, 8oz cup | 29mg |
The European Food Safety Authority puts the safe limit for healthy adults at 400mg a day and 200mg in a single dose, or 200mg a day if you're pregnant. A single grande Blonde Roast carries more caffeine than two cans of Monster and goes well past that single-dose line.
Sources: European Food Safety Authority, 2015; Mayo Clinic; Starbucks; brand labels.
Why the daily limit splits by genotype
Timing is half the picture. The other half is how much you drink across the whole day, and a slow metaboliser is still carrying the morning's caffeine when the next cup arrives. A 2006 study of 4,028 people in Costa Rica linked 2 to 3 cups a day to better heart outcomes in fast metabolisers, with 22% lower odds of a first heart attack. In people carrying a C, the same 2 to 3 cups went with 36% higher odds, rising to 64% at four or more cups.
Source: Cornelis et al., JAMA, 2006.
If you train: your pre-workout, decoded
The same gene shapes what caffeine does for performance. In 2018, researchers gave 101 competitive male athletes caffeine or a placebo before a 10km cycling time trial, then split the results by CYP1A2 genotype.
- AA riders were 6.8% faster at the higher dose of 4mg per kg of body weight, and 4.8% faster at 2mg per kg.
- AC riders saw no effect.
- CC riders were 13.7% slower at the higher dose.
Source: Guest et al., 2018.
Same dose, and the CC riders got slower. If you're CC, take the caffeine out of your pre-workout and compare your times over your next few sessions. If you're AA, a moderate dose before a hard session is worth keeping, as long as it lands outside your sleep window.
Find out which one you are
AA, AC or CC? Your DNA already knows. Myoform reads all 3 billion base pairs of your genome, rs762551 included, then builds your supplement blend around what it finds. Get your genetic test.
The Decode is for education only and isn't medical advice.