“Although the body is very intelligent, it cannot tell the difference between an actual situation and a thought. It reacts to every thought as if it were a reality. It doesn’t know it is just a thought. To the body, a worrisome, fearful thought means “I am in Danger,” and it responds accordingly, even though you may be lying in a warm and comfortable bed at night. The heart beats faster, muscles contract, breathing becomes rapid. There is a buildup of energy, but since the danger is only a mental fiction, the energy has no outlet. Part of it is fed back to the mind and generates even more anxious thought. The rest of the energy turns toxic and interferes with the the harmonious functioning of the body.”
Quote from: A New Earth by Eckhart Tolle.