Many animals need sleep. But all of the threats and demands animals face don’t just go away when it’s time to doze. That’s why a range of birds, mammals, and even humans experience some degree of asymmetrical sleep, where parts of the brain are asleep and other areas are more active. So, how does it work? Masako Tamaki explores how animals' brains remain vigilant even at their most vulnerable. Lesson by Masako Tamaki, directed by Biljana Labović. Animated by Iva Ćirić, designed by Manja Ćirić.