AbstractTwo theories on the cause of insomnia have emerged, the somatic and the cognitive. To test the cognitive theory, 15 insomniac and 15 normal sleepers were invited into the sleep laboratory. In the last of three sessions ostensibly designed to measure normal presleep physiological processes, a polygraph malfunction was staged raising the possibility of accidental shock. The major outcome was that insomniacs showed a significant increase of skin conductance level during this session while normals relaxed even further. This probably reflected the presence of anxious ruminations for insomniacs and the absence of such for normals in response to the third night threat. Insomniacs' tonic level of frontalis muscle tension surpassed normals but was non‐reactive to the third night threat, suggesting a minor role for somatic arousal in insomnia. The present results support the primacy of cognitive arousal in insomnia and invite emphasis on cognitive control treatment strategies for this sleep disorde