Why do we sleep?
D000818:Animals; D001921:Brain / Q000166:cytology / Q000502:physiology; D006801:Humans; D008568:Memory / Q000502:physiology*; D009474:Neurons / Q000166:cytology / Q000502:physiology; D012890:Sleep / Q000502:physiology*; D012892:Sleep Deprivation / Q000503:physiopathology*; D012895:Sleep, REM / Q000502:physiology*