A descriptive analysis of sleep and wakefulness states during maternal behaviors in postpartum rats.

Mother rats spend most of their time nursing their litter during the early stages of the postpartum period, only occasionally leaving the nest. The suckling stimulus from the pups elicits the adoption of nursing postures, during which milk ejection occurs, an event associated with the occurrence of non-REM (NREM) sleep in the rat. Despite this evidence, the characteristics of sleep during different nursing postures along the postpartum period remain unknown. The present study aims to describe the sleep pattern of mother rats while nursing, hovering over their pups and when being away from the pups. For this purpose, lactating females were implanted with electrodes for chronic polysomnographic recording. Simultaneous recordings of sleep-wakefulness cycle and maternal behaviors were performed in both the light and dark phases of the first and second postpartum weeks. Results indicate that while mothers were most of the time awake when hovering over pups and when staying away from pups, they mainly remained in NREM sleep when adopting low kyphosis posture, the most common nursing posture. The sleep-wake pattern during most maternal behaviors was quite stable between the light and dark phases of the first and second postpartum weeks. In addition, the sleep fragmentation was higher during the nursing bouts compared to that observed when mother rats slept without the pups, but sleep depth did not differ between these behaviors. Our results provide an original description of how mother rats synchronize their own sleep-wakefulness cycle with the maternal care of the pups during the postpartum period.