Insomnia symptoms (difficulty initiating sleep, difficulty maintaining sleep, and early morning awakening) are common, with a prevalence of 20% in the Japanese general adult population. In this paper, the author describes the clinical steps in the differential diagnosis of insomnia symptoms for readers who are not specialized in sleep medicine. When patients complain of insomnia symptoms, physicians first need to check their sleep habits and optimize them if they are inappropriate and interfere with good sleep. Medical history and medication use should also be reviewed in the first step because some medical conditions (e.g., articular diseases, skin diseases, cardiopulmonary disorders) and medications (e.g., steroids, interferon, methyldopa, levodopa) can cause insomnia symptoms. Insomnia symptoms can be caused by a variety of sleep disorders other than insomnia disorders, such as obstructive sleep apnea, restless legs syndrome, periodic limb movement disorder, and delayed sleep-wake phase disorder. A diagnosis of insomnia disorder is given only if the insomnia symptoms cannot be fully explained by another sleep disorder.