Sleep researchers need to bring Darwin on board: elucidating functions of sleep via adaptedness and natural selection.

The development of neural multifunctionality - given brain regions carrying out more than one function - conferred great efficiency on brain function at early stages of evolution. This applied to animals that led relatively simple lives with few needs for long-term memories, such as many lower invertebrates - many molluscs, echinoderms, worms, etc. As more complex lifestyles and detailed focal vision evolved, needs for self-initiated and reflexive activities increased in frequency, and recognition of many locales, conspecifics, and other forms of life became essential. These developments were accompanied by greatly expanded needs for neural processing supporting sensory and motor activities, and establishing and storing long-term memories. Since these categories of neural processing occur in largely overlapping brain regions, brain functioning would have become increasingly maladaptive, had the evolution of these more complex lifestyles not been accompanied by compensating adaptations that obviated these potentially conflicting brain activities. These adaptations consisted of: first, restful waking; second, primitive sleep; and finally, fully developed sleep, with its specialized rapid-eye-movement and non-rapid-eye-movement states, that contribute to the maintenance of great efficiency of brain function. The only animals with detailed focal vision that can achieve highly efficient brain function without sleep, are those in which demands on memory processing are greatly reduced in consequence of routine, monotonous, almost purely reflexive lifestyles, with few needs for acquiring experiential long-term memories. The best known animals in this non-sleeping category are tunas and many sharks.