A hypothesis on the selective advantage for sleep
  In this note, we present a hypothesis for the emergence of the phenomenon of sleep in organisms with sufficiently developed central nervous systems. We argue that sleep emerges because individual neurons must periodically enter a resting state and perform various ``garbage collection'' activities. Because the proper functioning of the central nervous system is dependent on the interconnections amongst a large collection of individual neurons, it becomes optimal, from the standpoint of the organism, for these garbage collection activities to occur simultaneously. We present analogies with complex structures in modern economies to make our case, although we emphasize that our hypothesis is purely speculative at this time. Although the ``garbage collection'' hypothesis has been previously advanced, we believe that our speculation is useful because it illustrates the power of a general paradigm for understanding the emergence of collective behavior in agent-built systems. 
Emmanuel Tannenbaum