Though on average, one third of a person's life is spent sleeping, scientists do not yet fully understand this complicated physiological phenomenon. It is, however, clear that sleep is essential, not only for humans but for almost all animals. Sleep affects our daily functioning and our physical and mental health in many ways and problems with sleep profoundly disturb both sleeping and waking life. There are more than 70 different sleep disorders, generally classified into one of three categories: lack of sleep, disturbed sleep and excessive sleep. Learn more about the importance of sleep in human life and the many various disorders that disrupt it in this book.