The association between insomnia and sleep apnea has received little attention from health professionals in the past few decades. However, recent studies have shown a high prevalence of insomnia complaints in patients with objectively diagnosed obstructive sleep apnea (OSA) syndrome. In this paper we have reviewed data published on different aspects of this association: the clinical profile of sleep-disordered breathing (SDB)-plus, the nature of the association, the role in the onset of insomnia played by OSA itself and other comorbidity factors such as depression or the restless leg syndrome. Finally, we have reviewed data and hypotheses on the metabolic implications of OSA and insomnia, and we speculate on the role that hypothalamic–pituitary–adrenal axis hyperactivity may play in a hypothetical interrelation between OSA and insomnia. The apparent paradox implied by this clinical association reveals the need for interdisciplinary training for physicians who treat both types of disorders.