Why does coffee after lunch ruin my sleep but not my friend's?
The short answer
Around half of all adults are slow caffeine metabolisers, and for them the CYP1A2 gene keeps caffeine active in the body for up to ten hours instead of four.If you are a slow metaboliser, a 2pm coffee can still be circulating at midnight. Your friend with the fast variant has cleared the same cup by dinner. Same drink, opposite night.
A second gene, ADORA2A, decides how strongly you feel caffeine even if you clear it quickly. Between the two, some people simply cannot have afternoon coffee without paying for it at night.
Do this today
Cut all caffeine after noon for one week. If you fall asleep noticeably faster, you have your answer about your own wiring.
Genes in playCYP1A2ADORA2A