SUMMARY In order to evaluate daytime arousal in patients with persistent insomnia, electrodermal activity (EDA) was measured in a habituation paradigm (21 tones, 80 dB, 1s) in forty patients with persistent insomnia and in twenty normal subjects. Results showed that the patients had higher EDA than the controls implying a higher arousal level in the patients. Among the patients the distribution of trials to a habituation criterion was bimodal and patients were separated into habituators (n= 24) and non-habituators (n= 16). Non-habituators had significantly higher EDA than habituators and they also had a shorter nocturnal sleep time. Thus, daytime arousal was significantly related to nocturnal sleep in the patients. Both patients with Psychophysiological Insomnia and Insomnia Associated with Psychiatric Disorders had higher EDA than the controls, and EDA was also significantly related to sleep in both groups. Results lend further support for the notion of an excessive daytime arousal, which may serve as a common predisposing factor in the initiation or maintenance of persistent insomnia.