I won't say nobody ever engineers an excuse (not even me ;)), but personally I feel better by just being truthful about it, I've never had a bad response to that. And I never give anyone a bad time about them cancelling either. Also, I just try to not plan too much anyway, because I know by now I need a certain amount of downtime, seems like it may be a bit more than most.

We're all human, and we all need downtime, no shame or guilt in admitting that.