Sleep is the state of natural rest observed in humans and throughout the animal kingdom, including in all mammals and birds, and in many reptiles, amphibians and fish
In humans, other mammals and many other animals which have been studied, such as fish, birds, mice, ants and fruitflies, regular sleep is necessary for survival
People born with the rare genetic disorder Morvan’s fibrillary chorea or Morvan’s syndrome can go without sleep for several months at a time. Michel Jouvet and his colleagues in Lyon, France, studied a 27-year-old man and found he had virtually no sleep over a period of several months. During that time he did not feel sleepy or tired and did not show any disorders of mood, memory, or anxiety. Nevertheless, nearly every night between 9:00 and 11:00 p.m., he experienced a 20 to 60-minute period of auditory, visual, olfactory, and somesthetic (sense of touch) hallucinations, as well as pain and vasoconstriction in his fingers and toes.[8] In recent investigations, Morvan's syndrome has been attributed to serum antibodies directed against specific potassium (K+) channels in cell and nerve membranes.
A dream is the experience of a sequence of images, sounds, ideas, emotions, or other sensations during sleep, especially REM sleep
REM sleep occurs in all mammals and birds. It appears that the amount of REM sleep per night in a species is closely correlated with the developmental stage of newborns. The platypus for example, whose newborns are completely helpless and undeveloped, has 8 hours of REM sleep per night; in dolphins, whose newborns are almost completely functional at birth, almost no REM sleep exists after birth.