The presence of insomnia in patients with sleep apnea seems paradoxical as excessive sleepiness is one of the major symptoms of sleep apnea. However, recent research has shown that about half of patients with sleep disorder breathing experience insomnia. Moreover, patients complaining of insomnia or non-restorative sleep may also present with moderate to severe sleep apnea syndromes. Thus, in recent years, clinicians have become more aware of the possible association between insomnia and sleep apnea. This article reviews data published on different aspects of this co-occurrence.