The problem of defining depth of sleep.
D000818:Animals; D001341:Autonomic Nervous System / Q000502:physiology; D001519:Behavior; D002415:Cats; D002540:Cerebral Cortex / Q000502:physiology*; D003243:Consciousness; D004569:Electroencephalography; D005133:Eye Movements; D006801:Humans; D009043:Motor Activity / Q000502:physiology; D012890:Sleep / Q000502:physiology*; D014851:Wakefulness / Q000502:physiology