What the Karolinska scale measures
The Karolinska Sleepiness Scale, or KSS, is a subjective measure of situational sleepiness. The person reports how they feel at that moment, considering the recent period. It is used in research on sleep, shift work, driving and performance. It does not measure exactly the same thing as muscular tiredness or, on its own, summarise sleep quality over previous weeks.
A momentary measure
Its usual format uses nine ordered levels from greater alertness to greater sleepiness. An authorised version appropriate to participants’ language should be used, with its instructions unchanged. This entry explains the concept without reproducing the questionnaire. Changing labels, adding levels or modifying the reference period makes comparison with studies or earlier measurements difficult.
What it contributes to prevention
It can help establish how sleepiness changes during a working day or under different shift arrangements. The information becomes more useful when recorded with the time, work duration, breaks and task context. Its preventive purpose is to identify conditions that reduce alertness and guide organisational changes, rather than place all responsibility for rest on the worker.
What validation research shows
Research has compared the KSS with performance and electroencephalographic measures. Associations have been observed between greater reported sleepiness and changes in those measures. This supports its usefulness as a subjective indicator, but does not turn each response into an exact prediction of an accident. A study’s validity also does not eliminate individual differences, context or the need for complementary methods.
Limitations and confidentiality
The KSS does not diagnose sleep apnoea, replace clinical assessment or provide automatic permission to continue a critical task. An apparently favourable score also does not demonstrate an absence of fatigue. There should be a procedure for reporting sleepiness and taking action without penalising the report. Personal data should be collected proportionately and group results presented without identifying individuals.
Hypothetical example
A team examines whether the final part of a night shift involves greater sleepiness. It records KSS at defined times and reviews the results alongside breaks, workload and opportunities for recovery. The decision may be to reorganise a demanding task or strengthen handover arrangements. The effect is then checked. The scale informs that decision, while risk management remains the organisation’s responsibility.
