The person asking you to take more should understand what you are already holding.

Nobody can allocate work well from outside your week. What you are already carrying is largely invisible: commitments made to other people, difficulty at home, the thing that always takes longer than it looks. A request made without that picture is less unfair than uninformed, and saying yes without correcting it protects nobody for long.

When someone asks you to take more, describe what you are holding before you answer, in specifics rather than in general tiredness. Ask which of these should move to make room. Let the person asking make the trade with real information instead of leaving it to you alone.