From the TED Blog:

“When you have a team of passionate, dedicated overachievers, you don’t need to push them to work harder, you need to help them rest. By taking the same two weeks off, it makes sure everyone takes vacation,” she said. “Planning a vacation is hard — most of us still feel a little guilty to take two weeks off, and we’d be likely to cancel when something inevitably comes up. This creates an enforced rest period, which is so important for productivity and happiness.”

Seemed weird when I first heard of this, but it really is efficient and effective. More organizations should do this.