Mammalian sleep may have no adaptive advantage over simple activity-rest cycles.

The adaptive value of sleep remains unknown in spite of the intense research performed throughout the last decades. However, few sleep researchers are aware of the difficulties posed by the blind acceptance of an extreme adaptationist viewpoint. Under this philosophy, every anatomical and functional detail present in a living being should have a positive adaptive value, a position that has been considered as rather doubtful. In this report, it is proposed that most of the physiological changes used for mammalian sleep definition could be mere by-products of other true adaptations, such as the ontogenetic and phylogenetic development of the nervous system. As a result, complex mammalian sleep could have no adaptive value over that of the simplest forms of rest-activity cycles present in all living forms. In addition, it is proposed that the absence of adaptive value should, by default, be the first option regarding the function of sleep. Besides, the burden of the proof should be always charged over the proponents of every particular adaptive function. As this proof has not been reached, it is the absence of function for sleep which should be taken for granted.