Cambridge Discovery Education Interactive Readers are the next generation of graded readers - captivating topics, high-impact video, and interactive exercises designed to motivate and engage. Why do we need sleep? What if we get too little or too much? Explore the mysteries of the night - like what really happens when we sleep! Use your unique code inside the book to access the online reader, complete with videos and interactive exercises. Word count: 1679. Lexile reading level: 640L.