Purpose: The study was conducted to examine the factors which affect the insomnia of nurses working in the intensive care unit of a university hospital, and to determinate insomnia severity. Methods: Although there are few studies evaluating nurses' insomnia, it is an important issue worldwide. The research was conducted with a cross-sectional and descriptive design with 149 nurses working in the Intensive Care Units of a university hospital. The data were collected with Data Collection Form and Insomnia Severity Index with face-to-face interview method. Descriptive methods and Chi-Square Test were used for statistical analyses, and p<0.05 value was considered to be statistically significant. Results: It was determined that the mean age of the participants was 31.04±5.93, 54.4% worked at surgical Intensive Care Units, many of them had a weekly working duration of 40-48 hours, and 62.4% worked in shifts. It was also determined that 75.2% of the nurses experienced insomnia. The difference between gender, weekly working duration, working style, sleep onset latency, sleeping duration and insomnia severity status was found to be statistically significant (p<0.05). No statistically significant differences were detected between nurses’ smoking status, drinking alcohol status, drinking coffee status, daytime coffee consumption, evening coffee consumption, tea drinking status, daytime tea consumption, evening tea consumption, sports and doing sports frequency, and insomnia severity (p>0.05). Conclusions: It can be argued that the majority of nurses who participated in the study experienced insomnia and moderate insomnia, and this result may pose a threat for nursing care in Intensive Care Units.