Separation from the mother disturbs infant sleep. For the brain to mature there needs to be sleep cycling. Supine sleep comes with no sleep cycling and in fact profoundly disrupts sleep cycling. Bergman claimed that a baby lying on his back may look like he is sleeping but he really isn’t because how can anyone sleep when in a stressed state!
He explained that with supine sleep, you get parasympathetic shut down and the parallel increase in sympathetic tone, which does not happen normally, keeps the baby awake and increases REM sleep, which does not allow for sleep cycling and resulting brain maturity. Separation disturbs sleep and you get a cortisol effect.
Dr. Bergman has a way of saying something so we won’t forget it. He mentioned that no mammals lie their babies on their backs to sleep! As humans are mammals, it is not natural for us to do that either.