The Insomnia Severity Index (ISI) is a brief, validated psychometric tool designed to quantify the perceived severity of sleep difficulties. Developed by Dr. Charles M. Morin and colleagues, the ISI is not a diagnostic instrument but rather a measure used to assess the impact of sleep problems on an individual’s life. The purpose of this tool is to provide a standardized, subjective assessment of a person’s experience with insomnia over the preceding two weeks. Since the ISI is a short, self-report questionnaire, it is widely used in both clinical settings and research.
Understanding the Seven ISI Questions
The Insomnia Severity Index uses seven distinct questions covering the full spectrum of insomnia experience, grouped into three core domains: night-time symptoms, satisfaction with sleep, and daytime impairment. Respondents rate the severity of their problems over the last two weeks.
The initial three questions focus on nocturnal aspects, addressing difficulties initiating sleep, staying asleep, and waking too early. The remaining four questions assess the psychological and functional impact of poor sleep, including satisfaction with sleep, interference with daily functioning, and the degree of distress the problem causes.
Each of the seven questions is rated using a five-point Likert scale, with numerical responses ranging from 0 to 4. A score of 0 indicates “None” or “Very Satisfied,” while 4 signifies “Very Severe” or “Very Dissatisfied.” This standardized scaling provides a quantitative measure of subjective feelings.
Calculating Your Insomnia Severity Score
The total Insomnia Severity Index score is calculated by summing the responses from the seven questions. After answering each question with a score between 0 and 4, the individual adds these seven numbers together to arrive at a single total score, which provides a metric for the overall perceived severity of the sleep problem.
Since each of the seven items has a maximum score of 4, the lowest possible total score is 0, indicating no sleep problems. Conversely, the highest possible total score is 28, representing the maximum perceived severity across all seven aspects of the index.
What Your ISI Score Means
The total ISI score corresponds to one of four established severity categories, providing a clear interpretation of the self-assessed sleep problems.
- Scores 0–7: No clinically significant insomnia. Sleep patterns are generally normal, and professional intervention is typically unnecessary.
- Scores 8–14: Subthreshold or mild insomnia. The individual experiences noticeable, but not severe, sleep disturbances. Individuals scoring in this range might consider simple behavioral adjustments or sleep hygiene improvements.
- Scores 15–21: Moderate insomnia. This suggests significant sleep difficulties impacting daily life and functioning. Consultation with a sleep specialist or healthcare professional is advisable.
- Scores 22–28: Severe insomnia. This points to persistent and profound sleep issues that significantly impair the individual’s quality of life.
For those scoring 15 or higher, the results suggest a level of severity where formal treatment, such as Cognitive Behavioral Therapy for Insomnia (CBT-I), should be strongly considered. The higher the score, the more urgent the need for professional evaluation and support.
How Doctors Use the Insomnia Severity Index
Clinicians utilize the Insomnia Severity Index as a robust outcome measure to guide and monitor treatment, not for formal diagnosis. The ISI is an efficient screening tool during initial consultations to quickly assess the subjective severity of symptoms. This initial score establishes a baseline against which future progress can be measured.
The primary application of the ISI in a clinical setting is monitoring the patient’s response to an intervention, such as medication or behavioral therapy like CBT-I. By having the patient complete the index at regular intervals, the doctor can track changes in the total score over time to determine if the chosen treatment is effective.
In clinical research, a reduction of 6 points or more on the total ISI score is frequently regarded as a minimal clinically important difference, signifying a meaningful improvement in the patient’s condition. This 6-point change threshold confirms that the patient is experiencing a genuine benefit from the treatment. The ISI thus acts as a practical, quantifiable metric to assess treatment efficacy and make data-driven decisions about continuing or modifying a patient’s care plan.