Hibernation and Circannual Rhythms of Sleep

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Abstract
Euthermic ground squirrels (Citellus lateralis) maintained at an ambient temperature of 22 C and on a 12L/12D photoperiod exhibit a circannual rhythm of sleep with its maxima coinciding with the hibernation season. Percent time asleep increased in the fall, remained high during the winter, decreased in the spring, and reached its lowest levels during the summer. The proportions of sleep time occupied by slow-wave sleep (80%) and rapid-eye-movement sleep (20%) remained constant throughout the year.