Have you ever felt tired one moment and then, without resting, have the tiredness disappear? Similarly with a feeling of sickness? Some people tend to indulge the slightest suggestion of tiredness or sickness and make a big deal of it. Others are more likely to shake the suggestions off and keep going. These are clearly mental suggestions of tiredness and sickness, and they are devilish, not our friends.
The Bible says, "Resist the devil, and he will flee from you." Science and Health with Key to the Scriptures states, "Stand porter at the door of thought. Admitting only such conclusions as you wish realized in bodily results, you will control yourself harmoniously."
What is it that is to resist and to stand porter at the door of thought? Not simply thinking, for then we have are stuck with a battlefield of good thoughts and bad thoughts warring with each other, a battle of will, if you will! No. It is more than thinking. We suggest it is consciousness that resists the devil and stands porter at the door of thought.
Consciousness? That awareness which is above thinking. In its purest form, it is the great I AM. But apparently, we are all at different states and stages, and the process of spiritual growth is the process of eliminating the negative and limited, and purifying consciousness.
Practically speaking, I find it helpful to recognize that consciousness is never tired, never sick, never separated from itself, but always in a state of dominion if I will but use it! If the mental suggestion seems overwhelming, and I do need to take some time off, I never consider it a sick day, but rather a mental health day - a time to replenish my oil, to commune with Spirit, read or listen to some Psalms, etc. This works for me, so maybe it will work for you.