"Remember the sabbath day, to keep it holy."
Refreshment is similar to rest. Rest is commanded of us once every seven days - the Lord’s day. Even God Himself rested. So I can know we do need rest and refreshment.
It seems to me the line between when I'm selfish versus needing refreshment is focus. Either way - whether I actually need rest, or am being self-focused - my focus needs to be on Christ, and His will and glory. The question to ask seems to be: why am I seeking rest? If I seek rest because I've begun to focus on needing rest, rather than accomplishing God’s will, my focus needs to change. After this, I can determine whether I truly need rest (and then I can get it!).
Proverbs 3:7-8 says, “Do not be wise in your own eyes; fear the Lord and turn away from evil. It will be healing to your body and refreshment to your bones.” My focus on Him (more specifically, being humble under Him and placing my fear upon Him) produces healing and refreshment.
Most often, I go on vacation seeking rest, and inevitably come back disappointed and still needing refreshment... because my focus is on myself. My expectations and my needs and desires weren’t met. In my self-focus, I am prideful, not humble. Self-serving, not God-serving.
When I go looking for what the Lord is going to do that day, I inevitably come back refreshed. My focus is on His will, and what He would have me do. He is in control, and whatever I can't get done doesn't matter. God-serving, not self-serving.
It is for this reason (among others, I’m sure) God commanded the day of rest to be His day. We achieve physical rest by resting physically (hence a day without work), but we receive refreshment by exalting Him rather than ourselves.
To go back to the example of the slave: were I that person, I may not get much rest. Yet I can still get refreshment by my humility and fear of the Lord. Physical rest may not be possible, but that simply means God has determined it isn’t best for me and/or those I affect right now, else God would have made rest possible. Talk about a test in faith, patience, humility, and endurance! What a testimony of God’s glory and strength to be refreshed without rest!
May I live in the constant awareness of my humble place under the mighty hand of God. (1 Peter 5:6)