In this chapter, we follow the journey of a teenager, Sophia, who was struggling with excessive daytime sleepiness that significantly impaired her ability to function at school and in social situations. Seeking answers, she consulted a sleep specialist, and through a comprehensive evaluation, physical examination, symptom tracking, and diagnostic tests, she was ultimately diagnosed with idiopathic hypersomnia—a sleep disorder that often goes undiagnosed. Using her story as a foundation, this chapter guides the reader through the process of conducting a comprehensive sleep evaluation, highlights screening tools that aid in information gathering, and outlines the workup for excessive daytime sleepiness. It also covers the diagnostic criteria for hypersomnolence disorder and idiopathic hypersomnia while delving into the neuroanatomy and pathophysiology of the disorder. Finally, the chapter provides an overview of effective treatment strategies, demonstrating how accurate diagnosis and personalized care can help individuals like Sophia regain control of their lives.