It really depends on where your reading and what your reading.
Reading is action characterized by rhythmic eye movement, a relaxed physical position and a focused attention. When you’re in this state, your body becomes passive and your alertness decreases unless the topic that you’re reading requires you to use your mind like mathematical calculations, deductive thinking, reasoning etc. Reading is tiresome. When reading continuously, your body exerts a lot of energy from your eyes as magnetic waves. Mental work is three times more demanding than physical work.