It is a fundamental concept of yin and yang. In this instance, yang is activity and yin is rest.
Yang, by it's very nature, will eventually expend itself. That goes for people, too. We need to rest. And if we are yang for too long, we eventually become exhausted and collapse (yin).
The best yin is deep and restful sleep. This essential part of life is often ignored as our modern lifestyle encourages us to work and play hard. There's so much to distract us, when everything is available 24-hours a day.
Power napping can really help to top up your batteries. If you do not think you have the discipline, you could try other methods. There is a product I have used for many years to help this - Recharge (I'm not on commission). This teaches you a simple method to quickly recharge your batteries. It is not a substitute for sleep, but can help you if you feel burned out.
Randy Pausch was an American University lecturer. He was diagnosed with pancreatic cancer (which is inoperable), and this was his last lecture, entitled "Achieving your childhood dreams.". It's over an hour and sixteen minutes long, so make sure you have time to watch.