Karolinska Sleepiness Scale (KSS)
Updated October 8, 2026 · 2 min read
The Karolinska Sleepiness Scale (KSS) is a 9-point rating of how sleepy you feel right now, from 1, "extremely alert", to 9, "extremely sleepy, fighting sleep". Researchers at the Karolinska Institute in Stockholm published it in 1990, and it is widely used in studies of shift work, sleep loss and driving.
The nine steps, with a night-shift example
You pick one number from 1 to 9 for how sleepy you feel right now. 1 is extremely alert, 5 is neither alert nor sleepy, and 9 is extremely sleepy and fighting sleep; a higher number means sleepier.
Say you rate yourself three times on a 23:00 to 07:00 ED night: 3 at 23:30, 6 at 03:00 and 8 at 05:30. Each rating covers the last few minutes, so the series shows sleepiness building toward the circadian low in the early morning.
Original labels and the later version
The 1990 scale labeled only the odd steps, 1, 3, 5, 7 and 9, and raters using it are encouraged to use the unlabeled steps too. A 2001 driver sleepiness study added labels to 2, 4, 6 and 8. When a 2016 study compared the two versions in 12 adults kept awake for 24 hours, the ratings were very similar.
The scale's author asks that it not be reproduced or adapted without written consent, so this page describes it rather than printing it as a form. The 2016 comparison paper, which is free to read, shows both versions with every label.
What the numbers mean
- In the 1990 study, signs of sleep in brain waves and eye movements appeared reliably only once ratings fell between "sleepy" (7) and "extremely sleepy, fighting sleep" (9).
- A 2014 review led by one of the scale's authors linked ratings above 6 with impaired driving and with sleep intrusions in brain-wave recordings.
- Ratings rise with sleep loss and with night work, including night driving.
- Context moves ratings: physical activity, social contact and light lower them by 1 to 2 points, and a monotonous task raises them by 1 to 2.
- A 2014 airline study treated 7 or more as severe sleepiness.
That context effect cuts both ways on a shift. A busy department may hold your rating down, and a quiet drive home after a night may push it up.
KSS and the alertness model
Predicted alertness from the three-process model converts to an approximate KSS rating: 10.6 minus 0.6 times the alertness score. The model's sleepiness line of 7 works out to about KSS 6.4. The shift alertness calculator shows this estimate for each shift's lowest point.
Common questions
- What time span does a KSS rating cover?
- The last few minutes. The 2016 study describes the immediately preceding 5 minutes; a sleep-scale handbook gives the last 10.
- Can the KSS diagnose a sleep disorder?
- No. It measures sleepiness in the moment, which changes with time of day, prior sleep and activity. A sleep-scale handbook notes that it does not measure long-term (trait) sleepiness, so it has seen little clinical use.
- How is it different from the Samn-Perelli scale?
- The KSS rates sleepiness on 9 points; the Samn-Perelli scale rates fatigue, or tiredness, on 7, with wording from a 1982 US Air Force report.
See your own shifts this way.
Bring in your schedule from any scheduling app with its calendar link. Capillary charts your hours, nights, quick returns and hardest shifts. Only you can see it.
Sources
- Åkerstedt T, Gillberg M. Subjective and objective sleepiness in the active individual. Int J Neurosci 1990
- Åkerstedt Miley A, Kecklund G, Åkerstedt T. Comparing two versions of the Karolinska Sleepiness Scale (KSS). Sleep Biol Rhythms 2016
- Åkerstedt T, et al. Subjective sleepiness is a sensitive indicator of insufficient sleep and impaired waking function. J Sleep Res 2014
- Shahid A, et al., eds. STOP, THAT and One Hundred Other Sleep Scales. Springer 2012: Karolinska Sleepiness Scale
- Ingre M, et al. Validating and extending the three process model of alertness in airline operations. PLoS One 2014