The ontogenesis of sleep states, thermogenesis, and thermoregulation in the virginia opossum

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Abstract
Polygraphic recordings of sleep and wakefulness were obtained over 4‐hr periods from infant opossums, aged 48–79 days, while they were in the mothers' pouches. Concurrent body temperature measurements were obtained from littermates held at ambient temperatures of 25 and 30°C. The ontogenesis of sleep was similar to that of placental mammals: quiet sleep increased from 13% sleep time at 48 days to 55% at 79 days; conversely, active sleep decreased from 87% to 45%, respectively. The amplitude of the electroencephalogram (EEG) during quiet sleep increased with age and quiet sleep could be categorized as slow wave sleep at 75 days. Active sleep could be identified as rapid eye movement sleep at the same age. Temperature measurements revealed that thermogenesis appeared at approximately 55 days and the 1st signs of thermoregulation were apparent at 67 days. Near‐adult thermoregulatory capabilities were present by 79 days. Increases in percent time spent in quiet sleep corresponded with increases in percent adult thermogenesis. The appearance of slow wave EEG activity coincided with the onset of thermoregulatory capabilities. We suggest that lowered metabolism associated with slow wave sleep is adaptive in offsetting the increased energy demands of thermogenesis.