Why do babies smile in their sleep? This warm, intriguing question opens a window into newborn biology, early relationships, and the developing brain. This reassuring guide explains the difference between reflex smiles, spontaneous expressions during active sleep, and the socially responsive smiles that emerge as infants become more alert and connected. It explores what can and cannot be inferred about dreams, why facial movements change across the first months, and how feeding, comfort, wakefulness, and sensory experience shape early expression. Readers gain practical ways to observe without overinterpreting, support secure interaction, record patterns, and know when a persistent concern deserves discussion with a pediatrician or health visitor. It also makes room for cultural difference, neurodivergent communication, parental uncertainty, and the full range of infant expressions—including neutrality and distress. Clear, compassionate, and evidence-aware, this book replaces myths with useful developmental context and helps families enjoy the small moments that build connection. Part of The Smile and Happiness Library.