You can just say no. There is no need for an apology or explanation. If you are being inconvenienced or made to feel uncomfortable, it is acceptable for you to put yourself first.
Consider the instances you want to just say no yet feel you need to explain. It may be because you want to spend time with your family or you are tired and need to recharge, and even possibly because it is not something you like. We explain it because we believe it is necessary or the ‘polite thing.’ Even your supervisor may appreciate a firm ‘no’ when they repeatedly change direction or add the latest thing to do.
No is no. We need to get comfortable with it by saying it and hearing it. To be clear, ‘no’ alone is far more powerful since it does not in any way become a maybe. It’s a clear-cut ‘no’!
