The Evolution of Sleep
Sleep behavior probably began when reptiles had to adjust to life on land. The sleep signs that they gave, such as changes in brain pattern frequency, were the beginnings of the mechanisms of sleep for mammals. During evolution the two major descendants of reptiles, birds and mammals, both developed the ability to maintain body temperature and the ability to sleep. These two evolutionary characteristics are related because in order to maintain body temperature animals must speed up their physiological systems and raise their metabolic rates. Because of these changes, they were looking for ways to conserve energy. Sleep is the perfect solution to this problem.
Sleep is vital to the welfare and health of living things and its evolution demonstrates just how important it is for survival and how specialized it is to each animal's needs. Not all animals sleep for the same amount of time each day, and not all animals sleep in the same ways. Due to evolution, there are birds that can sleep during flight, fish that sleep while swimming, and some animals that sleep with only one side of their brain at a time (Coren, 1996). Some other aspects of sleep evolution to consider are the factors that determine sleep patterns and the degree to which the human sleep pattern has not evolved quickly enough to fit the lives we live, which causes us to accumulate a sleep debt.