2.2.6 Insomnia Severity Index (ISI)
The ISI is a valid and reliable tool to assess perceived insomnia severity. It was developed by Bastien et al.17. The validity and reliability studies of the scale in the Turkish population were conducted by Boysan et al.18. It evaluates the severity of sleep onset, sleep maintenance, early morning awakening problems, satisfaction with current sleep pattern, interference with daily functioning, noticeability of impairment, and level of distress caused by the sleep problem. Each item is scored on a 5-point Likert-type scale and higher scores indicate greater insomnia severity.