An electric pump is used to carry water from the ground level to the top of your building. Could this pump be left running continuously for around 70-100 years? Never! It would break down within a few months. Like any machine it needs to be rested periodically.
The heart works exactly like a pump. In a day it drives around 7500 litres of blood through your blood vessels. Your heart will continue its pumping action for the rest of your life, beating one lakh times a day. Doesn't this body machine need rest? It does. The heart takes rest in between heartbeats. This total period of rest adds up to about 6 hours per day.
Your building's pump is not designed to rest in between its strokes. So it must be switched off to allow it to rest. But the heart cannot be switched off even for a minute. And so, Nature, the greatest of all designers, has introduced the 'in-between beats' rest for the heart.