“Although the body is very intelligent, it cannot tell the difference between an actual situation and a thought. Â It reacts to every thought as if it were a reality. Â It doesn’t know it is just a thought. Â 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. Â The heart beats faster, muscles contract, breathing becomes rapid. Â There is a buildup of energy, but since the danger is only a mental fiction, the energy has no outlet. Â Part of it is fed back to the mind and generates even more anxious thought. Â 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.