The body, The spirit
In order to continually walk in the spirit we must obey the universal law instituted by God.
One of those law is to take care of your physical body. It is true that the spirit is greater than the flesh, fact! but if we don't take care of our flesh while on earth the body will break down or die, if the body dies, the spirit must depart, that is certain.
God established a universal law that says our bodies need proper rest and nourishment, but many of us tend to nourish the mind, feed the spirit but we care less about the body.
Come to think of it God himself rested on the seventh day after the work of creation establishing these principle for us to follow.