title: [The evolution of sleep: birds at the crossroads between mammals and reptiles].
abstract: BACKGROUND
  The evolution of sleep is one of the mysteries surrounding the sleep. Since the discovery of the two phases in the mammalian sleep, NREM and REM, the sleep researchers have unsuccessfully tried to understand their origin and the causes of the sleep duality. Looking for an answer to these questions, the sleep of reptiles (the parental group of mammals) and birds (also descending from reptiles) has been studied. As a result, NREM and REM were found in birds but not in reptiles, and the question remains thus unresolved.
  
  
  CONCLUSIONS
  This review presents a critical analysis of the published sleep studies developed in birds and points to a possible cause of the conflict: considerable doubts can be cast on the existence of the two sleep phases in birds. If birds turn to have only one sleep phase, they would have a very similar sleep to that of reptiles. As a consequence, the two phases, REM and NREM, would be exclusive of mammals, and both would have appeared as a consequence of the evolutionary changes in the development of the brain from reptiles to mammals.
authors.affiliation: Laboratorio de Fisiología, Universitat de les Illes Balears, Palma de Mallorca, Spain.
authors.forename: C
authors.initials: C
authors.lastname: Garau
authors.forename: S
authors.initials: S
authors.lastname: Aparicio
authors.forename: R V
authors.initials: RV
authors.lastname: Rial
authors.forename: S
authors.initials: S
authors.lastname: Esteban
mesh_terms: D000818:Animals; D005075:Biological Evolution; D001717:Birds; D006801:Humans; D008322:Mammals; D010507:Periodicity; D012104:Reptiles; D012890:Sleep / Q000502:physiology*; D012894:Sleep Stages / Q000502:physiology; D014851:Wakefulness / Q000502:physiology