Sleep disturbance is common during and after pregnancy. Occurring alone or as part of psychiatric or physical illness, insomnia and poor sleep quality are increasingly recognized as risk factors in the precipitation or perpetuation of various perinatal psychiatric disorders. Early identification and management of poor sleep have the potential to improve the short-term and long-term outcomes of psychiatric and physical illnesses. In this chapter, we review historical clinical observations and studies on the association of serious mental illness with insomnia, followed by a discussion of the nosology of peripartum psychiatric disorders. We then describe the relationship between insomnia/sleep loss and common psychiatric disorders during and after pregnancy. Based on our synthesis of the literature and clinical experience, suggestions are offered for the assessment, follow-up, and treatment of common psychiatric disorders during pregnancy and the postpartum period. We conclude with suggestions for future research emphasizing the need to have prospective data on the trajectories of sleep changes among patients with preexisting psychiatric disorders and among those at risk of developing these illnesses during or after pregnancy.