Here is it seen as part of becoming an adult that you know your limit and don't overpromise. If you know you've had a full schedule already this week, and that you really need your friday-night on the couch, then you'd simply not set up anything that evening and ask if next week is also possible.

And if you really need to back out because of course nobody is always perfectly estimating their energy levels, you simply say it like it is: I'm tired, let's reschedule for some other time.

Respect is shown by not wasting other peoples time. Don't act, if we are really friends I'll see through you anyway, and I also respect that you really are tired and cannot come.