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The extent to which changes in ultradian and circadian rhythms (URs and CRs) reflect seasonal variations in pineal melatonin secretion was assessed in male Siberian hamsters transferred from long to short day lengths. The period of the locomotor activity UR increased from 2.5h in long days to 4.5h in short day lengths, but this and most other features of the short-day ultradian phenotype were unaffected...
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