Your schedule is territory. If you don’t claim it, somebody else will plant a flag.

Empty time does not stay empty. It reads as available to everyone around you, so the shift, the favor, the last minute plan all land there, and none of it asked whether you had something planned. Wanting the time is not enough. The time has to be visibly taken before someone else takes it.

Put the things that matter to you in the calendar with a start and an end, the same way a shift goes in. Practice, study, the hour you are not available. When someone asks, you are not refusing them, you already have something, and that is a much easier sentence to say.