The fetus sleeps in the uterus. It is also asleep when brought to life. The infant spends most of the first few months of his or her life asleep, not differentiating between day and night. The infant's sleeping rhythm is finally adjusted to night time. As a person's life continues, the number of hours he or she sleeps decreases. But, what is sleep? Does life stop during sleep? Or is it more of a break period? What are its benefits? What is the status of dreams during sleep? Many sciences have jointly attempt to answer the former, in addition to other, questions. The latter may be remotely or closely related to this complicated process, despite its apparent simplicity, represented by sleep. The book seeks to answer the previous, in addition to other questions, related to this complicated process that does not occupy too much of our attention.