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Babies Sleep Better
Sleeping with a familiar person smooths the transition from one sleep state to another and lessens a baby's anxiety. When he partially awakens he can be helped to resettle before he fully awakens. For older children an attachment object such as a "blankie" serves this purpose. However, Dr. Sears contends that a baby needs a person. Babies do not yet have object permanence. If they are awakened and their mother is not there they do not comprehend that she is just in another room.
Mothers Sleep Better
Most parents do sleep better with their baby (once they have a chance to get used to it) but this is especially so for the mother. She does not lie awake wondering if her baby is okay. Physical closeness causes a mother and baby to share sleep cycles. A baby usually wakes during light sleep and the mother is then likely to be in light sleep as well. She can settle the baby without her sleep cycles being seriously disturbed. Awakening from deep sleep is what leads to exhaustion. Because the mother is awakened less often from deep sleep she is less likely to feel groggy or sleep-deprived.
Breastfeeding is Easier
Nursing occurs without the baby or the mother becoming fully awake rather than the mother being awakened by a crying baby and going to her when they are both fully awake.
Prolactin Levels Increase
Prolactin is a hormone secreted by the pituitary gland. Prolactin levels increase with sleep, nursing and touching. It is often referred to as the "mothering" hormone since it seems to promote "mothering" behavior. It also causes the mother to feel more rested. Acceptance and tolerance levels seem to increase with co-sleeping and nursing. Dr. Sears wonders if this may be due to prolactin. Some researchers suggest that prolactin is responsible for the similarity of dream and sleep cycles and brain wave patterns between mothers and their nursing babies.