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This article takes stock of the present state of research on the physiology of dreams. For about fifteen years, studies about paradoxical sleep, and in its wake, studies about dreams, have been the object of numerous debates. Pioneers considered that state as a moment when dreams we can remember and talk about occur, but the discovery that cats could be alert while in deep sleep questioned previous beliefs. Following Freud, neuropsychoanalysts make dream the touchstone of their works and try? not very successfully so far? to identify the networks, the structures, the hormones which, during the dreaming sleep, call out libido, That, or repression. As for evolutionist psychologists, they advocate a selective and ancient pressure: hunters and gatherers allegedly lived in a state of permanent alertness that required a regular maintenance every night of neuronal circuits. All in all, these noisy reworkings raise more epistemological or historical problems than they solve; the dream remains inaccessible to the attempt at naturalisation.