McKenna, James J., Natural History
For as far back as you care to go, mothers have followed the protective and convenient practice of sleeping with their infants. Even now, for the vast majority of people across the globe, "cosleeping" and nighttime breast-feeding remain inseparable practices. Only in the past 200 years, and mostly in Western industrialized societies, have parents considered it normal and biologically appropriate for a mother and infant to sleep apart.
In the sleep laboratory at the University of California's Irvine School of Medicine, my colleagues and I observed mother-infant pairs as they slept both apart and together over three consecutive nights. Using a polygraph, we recorded the mother's and infant's heart rates, brain waves (EEGs), breathing, body temperature, and episodes of nursing. Infrared video photography simultaneously monitored their behavior.
We found that bed-sharing infants face their mothers for most of the night and that both mother and infant are highly responsive to each other's movements, wake more frequently, and spend more time in lighter stages of sleep than they do while sleeping alone. Bed-sharing infants nurse almost twice as often, and three times as long per bout, than they do when sleeping alone. But they rarely cry. Mothers who routinely sleep with their infants get at least as much sleep as mothers who sleep without them.
In addition to providing more nighttime nourishment and greater protection, sleeping with the mother supplies the infant with a steady stream of sensations of the mother's presence, including touch, smell, movement, and warmth. …