The aim of this study was to estimate the prevalence of insomnia, differences between males and females, and between two age groups, and to explore the factorial structure of the Arabic Scale of Insomnia (ASI). Samples of university students (n=500) and employees (n=457) of both sexes were recruited. Female university students obtained the highest mean total score on the ASI, followed by female employees, male university students, and finally male employees. The salient significant differences were between female students and male employees and between male and female employees. Despite the differences in the prevalence rate of insomnia between the four groups, they all agreed on that the least prevalence rate was for the item: “I feel depressed when it is time for me to go to bed, whereas the highest prevalence rate was for the item: “My interrupted sleep annoys me.” Principle components analysis yielded three factors for each group. Most of their names were not identical. The most salient finding of this study was the statistically significant sex differences in insomnia in favor of women. It was concluded that is it important to introduce counseling for those who obtained high scores on the ASI.