The Insomnia Severity Index (ISI) is a tool designed to evaluate the severity of a person's insomnia by assessing different aspects of their sleep experience. The ISI is based on seven specific questions that gauge difficulties in falling asleep, maintaining sleep, early awakenings, satisfaction with sleep, noticeability of sleep problems to others, worries about sleep issues, and the impact on daily functioning.

Each question offers a selection of responses that correspond to a numerical score:

- Difficulty falling asleep, staying asleep, and problems waking up too early over the past two weeks are categorized and scored as:
  - None = 0
  - Mild = 1
  - Moderate = 2
  - Severe = 3
  - Very severe = 4

- Satisfaction with the current sleep pattern is evaluated as follows:
  - Very satisfied = 0
  - Satisfied = 1
  - Moderately satisfied = 2
  - Dissatisfied = 3
  - Very dissatisfied = 4

- Noticeability of sleep problems in terms of life impairment, level of worry or distress about sleep issues, and interference with daily functioning are rated as:
  - Not at all = 0
  - Little = 1
  - Somewhat = 2
  - Much = 3
  - Very much = 4

The responses are converted into scores, which are then totaled to produce a score ranging from 0 to 28. This total score determines the severity of insomnia and is interpreted accordingly:

- A score between 0 and 7 suggests "No clinically significant insomnia."
- A score between 8 and 14 indicates "Subthreshold insomnia."
- A score between 15 and 21 denotes "Moderate insomnia."
- A score between 22 and 28 reflects "Severe insomnia."

By summarizing these responses into a total score paired with an interpretation, the ISI provides insight into the severity of an individual's insomnia and aids in understanding the extent of sleep-related issues.

The Insomnia Severity Index (ISI) is a clinical calculator used to assess the severity and impact of insomnia. It is applicable to patients who are suspected of having insomnia or those who have already been diagnosed with the condition. The ISI is a valuable tool in both clinical and research settings, as it helps to quantify the patient's perception of their insomnia, the severity of their sleep onset and maintenance difficulties, and the degree to which insomnia interferes with their daytime functioning. It can also be used to monitor treatment response in patients with insomnia. The ISI is not suitable for use in patients with other sleep disorders, such as sleep apnea or narcolepsy, as it specifically targets insomnia-related symptoms.