title: The biology of natural sleep in animals.
mesh_terms: D000220:Adaptation, Biological; D000818:Animals; D001522:Behavior, Animal / Q000502:physiology*; D001717:Birds / Q000502:physiology; D005399:Fishes / Q000502:physiology; D009425:Nesting Behavior / Q000502:physiology; D009799:Ocular Physiological Phenomena; D012679:Sense Organs / Q000502:physiology; D012890:Sleep / Q000502:physiology*; D013997:Time Factors