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 as it is observed in all mammals and birds and many reptiles, amphibians and fish. Sleep is defined as a state of unconsciousness from which, as opposed to coma, a person can be aroused. Sleep is not a passive process or "switching off" of body functions – it is believed to be important in many physiologic processes including the processing of experiences and the consolidation of memories. Learn more about the process and effects of sleep (or lack of it) on physical and mental health in this book.