The coffee thing
Why the same cup keeps one of you wired and lets the other sleep like a baby.
The two of youYou can't have coffee after 2pm. Your partner has one at 9pm and sleeps fine.
If coffee ruins your sleep, you are likely a slow caffeine metaboliser (the CYP1A2 gene), so caffeine stays active in you for up to ten hours. That evening cup is still buzzing at bedtime.
If your partner sips espresso at night and drifts off, they are a fast metaboliser, clearing caffeine in around four hours. By bedtime it is basically gone from their system.
Different coffee cut-off times for each of you, not one rule for the house. The slow one switches to decaf or chai-without-the-kick after lunch, the fast one keeps their evening cup guilt-free.
Whether coffee keeps you up depends on how fast your body clears caffeine, which is genetic, so two partners can react to the same cup in completely opposite ways.
About half of adults are slow caffeine metabolisers and half are fast. It is one of the most clear-cut everyday differences between two people, and it explains a surprising number of bedtime standoffs. The fast partner honestly cannot understand the problem, and the slow partner honestly cannot have that 6pm coffee. Both are telling the truth about their own bodies.