The standard you use on tired days should remember that you are tired.

Standards get set on good days and then applied on bad ones. The version of you that made the rule had slept and had energy to spare, and the version meeting it at the end of a hard week has none. When the standard cannot bend, tiredness starts to feel like a character flaw rather than a fact about the day.

Decide now what a tired day version of your important routine looks like, and let it genuinely count. Make it small enough that you would still manage it when things go wrong. Meeting the low bar and stopping is a better week than aiming high and abandoning it.