What keeps you up at night? Pondering deep questions? Excitement about a big trip? Stress about unfinished work? What if the very thing keeping you awake was stress about losing sleep? This seemingly unsolvable loop is at the heart of insomnia, the world’s most common sleep disorder. So what is insomnia? And is there any way to break the cycle? Dan Kwartler details the science of insomnia. Lesson by Dan Kwartler, animation by Sharon Colman.