Spot on.
I find it frustrating when I have what I perceive to be a lazy day. However, I accept that I can't be 100% productive 100% of the time. To be at your optimal best, rest is required at some point.
At the end of the day, if you're the type of person who is bothered by unproductive days, you're the same type of person who will make up for it in the future.