Insomnia disorder is defined as difficulties in initiating/maintaining sleep and/or non-restorative sleep accompanied by decreased daytime functioning, persisting for at least four weeks. For many patients suffering from depression and anxiety, insomnia is a pervasive problem. Many of the aetiological theories of insomnia postulate that heightened emotional reactivity contributes to the maintenance of symptoms. This review focuses on the role of emotional reactivity in insomnia, and how the relationship between insomnia and depression and anxiety may be mediated by emotional reactivity. Furthermore, studies investigating the valence of emotions in insomnia are reviewed. Overall, there is empirical evidence that dysfunctional emotional reactivity might mediate the interaction between cognitive and autonomic hyperarousal, thus contributing to the maintenance of insomnia. Moreover, dysfunctions in sleep-wake regulating neural circuitries seem to be able to reinforce emotional disturbances. It seems plausible that dysfunctional emotional reactivity modulates the relationship between insomnia and depression and anxiety. Considering the interaction between sleep and emotional valence, poor sleep quality seems to correlate with high negative and low positive emotions, both in clinical and subclinical samples. Good sleep seems to be associated with high positive emotions, but not necessarily with low negative emotions. This review underlines the need for future research on emotions in insomnia. (C) 2009 Elsevier Ltd. All rights reserved.