In this article, the evidence for hyperarousal in insomnia is reviewed, with an emphasis on neurobiological studies. Indicators of physiologic hyperarousal include electroencephalography-derived polysomnographic, autonomic, and neuroendocrine variables and outcome parameters of neuroimaging studies. Most studies documented an increased physiologic arousal in patients with insomnia, but it is not known whether physiologic hyperarousal is a cause or consequence of insomnia, what is the genetic basis of hyperarousal and what is the impact of early-life stressors and other life events, and what is the neurobiological basis of hyperarousal and how can it be modified in the most efficient manner.