Scientists have explored the question of why we sleep from many different angles. They have examined, for example, what happens when humans or other animals are deprived of sleep. In other studies, they have looked at sleep patterns in a variety of organisms to see if similarities or differences among species might reveal something about sleep's functions. Yet, despite decades of research and many discoveries about other aspects of sleep, the question of why we sleep has been difficult to answer. 

The lack of a clear answer to this challenging question does not mean that this research has been a waste of time. In fact, we now know much more about the function of sleep, and scientists have developed several promising theories to explain why we sleep. In light of the evidence they have gathered, it seems likely that no single theory will ever be proven correct. Instead, we may find that sleep is explained by two or more of these explanations. 

The hope is that by better understanding why we sleep, we will learn to respect sleep's functions more and enjoy the health benefits it affords. This essay outlines several current theories of why we sleep. To learn more about them, be sure to check out the "Bookshelf" feature at the end of this essay. There you'll find links to articles by researchers who are studying this fascinating question.