When it comes to work and rest, God's rhythm isn't America's. We celebrate 12-hour work days, short vacations, the ability to work on little sleep.
While having a busy week or two doesn't make you a sinner, I'm afraid we've missed the mark as a nation. Rest, leisure, time invested in relationship and community are of high value to God (and ironically, tend to make people and organizations more effective).
Is rest in your life rhythm? As a rule, it's a good a idea to have a bit of space each day for rest and a large dollop each week. This is God's design for humans.
If you avoid this in the name of productivity, you may want to reconsider, as it's probably working against you.
In the end, rest is not only good for you, but it shows that you trust God. It says "God, I know there are many things to do, but you're bigger than all my tasks. I can be still."