The days, the seasons, the years pass by... Our body can predict and prepare itself for the environmental changes. Predictability is very important to optimize its performance. However, how do our cells know when it is day, night, summer or winter? They need a signal, and that's where melatonin comes in. It is a hormone produced by a gland in the middle of our brain called pineal gland. Pineal gland only produces melatonin at night (and in the dark)! So melatonin is the sign our cells were waiting to tell them it is night. It turns out that in the modern world, we are overexposed to light. People work at night when they should be sleeping. Blue light coming from our TV, computer or smartphone can block melatonin production. Light exposure at night prevents our body to predict properly the environmental changes so it loses its rhythmicity. Disturbances of melatonin rhythms are associated with various health problems such as neurodegenerative diseases, heart disease, hypertension, and even cancer. See? Melatonin's actions go far beyond making you sleep ... and that's what I want to tell you in this book.