N-of-1, switchback, reversal, or dose-response

Test on yourself or one unit by switching the factor on and off and watching what tracks.

Every article says afternoon coffee wrecks sleep, your spouse agrees, and yet your sleep is so erratic anyway that you honestly can't tell. Studies describe averages across strangers. The question you care about is what coffee does to you, and you are a sample of one that you can actually experiment on.

The move is to compare safe conditions within the same unit, yourself, by switching the factor on and off and watching what tracks. Steps: alternate weeks with and without afternoon coffee while holding bedtime steady, log sleep quality each morning before checking which condition you're in if you can, then reverse again, and if you're curious, vary the dose (one cup versus two). The reversal is the punchline: if sleep improves off coffee, worsens when it returns, and improves again when you stop, that pattern is stronger evidence about you than any study. When it works, generic internet advice becomes your personal dose-response curve. Don't use it for anything unsafe to toggle, like prescription medication without your doctor, or for slow-moving outcomes that can't respond within a single switch.