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Four normal young adults underwent partial or complete inversion of their sleepwaking cycles to determine the relationship of the nocturnal release of human prolactin to sleep. Prolactin release shifted immediately and completely with shifts of sleep onset of 3, 6 and 12 hr. Thus, the nocturnal rise is dependent on the occurrence of sleep and is not based on an inherent rhythm related to time of day.
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Sassin et al. (1973) studied this question.
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