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Many researchers have attempted to characterize the association between sleep physiology and dreaming, with some consensus that both dream recall frequency and dream content fluctuate in predictable ways across a night of sleep. Dream recall and content seem to be tempered, on the one hand, by the cycle of sleep stages which occurs approximately every 90-minutes, termed the ultradian rhythm, as well as by the total duration of sleep and the time of night, linked to the circadian rhythm, the approximately 24-hour long cycle.