Our bodies and our minds are like tools or muscles. We use them to perceive the world external to our thoughts, and we use them to interact with the same external world. As with any tool or muscle, overuse without rest and repair, we can damage them...some times even beyond repair. So take care of your mind, which we tend to neglect. And take care of your body. It is not selfish to rest sometimes. It is virtuous because it helps you be a better person.