Reward a child’s distress with attention, they said (and say today), and you increase the probability of recurrence. A child left alone at night, with no human presence to “reward” him, eventually stops crying and makes do without. But sleep is not a reflex, like the canine salivation a flank steak provokes. The dozing adult brain rises and descends through half a dozen distinct neural phases every ninety minutes, in gradually lengthening symphonic movements that culminate in morning wakefulness. Sleep is an intricate brain rhythm, and the neurally immature infant must first borrow the patterns from parents.