A level 3 Oxford Bookworms Library graded reader. Written for Learners of English by Alex Raynham.

You're fast asleep, and nothing is happening. Or is it? In fact, your body is hard at work. Your lungs are taking oxygen from the air, and your heart is pumping blood round your body. Millions of pieces of information are travelling backwards and forwards to your brain all the time. Muscles are repairing themselves, and in your lymph nodes special cells are cleaning germs and waste from the body. You may think that nothing is happening, but in the extraordinary machine that is the human body, it is very busy indeed . . .