A 3-Hour Period Ultradian Rhythm of the Sleep-Wakefulness Cycle and Growth Hormone Secretion in the Immature Rat

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The temporal patterns of the sleep-wakefulness cycle and growth hormone secretion throughout a day in the male and female rat were studied in male and rats at 27 approximately 31 days of age. EEGs were recorded continuously from 27 to 31 days of age, and a time series of the amount of sleep for every 10 min was obtained. Serial blood sampling through an intracardiac cannula at 10-min intervals was performed for each of the 6-h periods beginning at 12:00, 18:00, 00:00 and 06:00 h. By analysis of the power spectrum and the least squares method, the time series was found to have approximately a 3.0-h periodicity in any of the 6-h periods and in either sex. The peak time which was estimated for the time series of GH concentrations was found to occur consistently around 02:00 approximately 03:00, 05:00 approximately 06:00, 08:00 approximately 09:00, 11:00 approximately 12:00, 14:00 approximately 15:00, 17:00 approximately 18:00, 20:00 approximately 21:00 and 23:00 approximately 24:00 h in either the male or the female rat. The time series of the amount of sleep over a 24-h period at 29 days of age revealed an ultradian rhythm with approximately a 3.0-h periodicity, and the peak time was estimated to be the same as that for the GH time series Taking the findings that the GH secretion is related to sleep in the prepubertal rat as described in the accompanying paper (Tsai, 1983), it could be speculated that the rat GH secretory rhythm develops by 30 days of age as a 3.0-h period ultradian rhythm that entrains to the time of day, relating to the rhythm of the sleep-wakefulness cycle.