Calibration of an objective alertness scale

• Systematic errors in mean and calibration of a subjective sleepiness scale are found.
• Validity of an objective sleepiness scale is established through its calibration.
• Evidence for possibility to correct subjective sleepiness self-scorings is provided.
• Steps of objective and corrected subjective scales show one-to-one correspondence.
• Steps are anchored to minimal/mild/moderate/marked/severe/disabling sleepiness levels.

In order to establish validity of an objective scale of alertness–sleepiness, one is forced to somehow calibrate it using subjective scales, such as the Karolinska Sleepiness Scale (KSS). We evaluated the effects of prolonged wakefulness on the extent of disagreement between objective and subjective KSS assessments, and tested whether calibration of an objective alertness–sleepiness scale can be established despite the limited reliability of subjective reports. Starting from 7 p.m., the resting electroencephalogram (EEG) was recorded at 2-hour intervals over the last 32–50 h of 44–61-hour wakefulness of 15 healthy study participants. Frontal and occipital scores on the 2nd principal component of the EEG spectrum and occipital alpha-theta power difference were computed for 1-min intervals of 5-min eyes-closed EEG recordings. To obtain alertness scale scores in the range from 5 to 0, positive and negative values of these EEG indexes were assigned to 1 and 0, respectively, and then summed. Although correlation between time courses of objective and subjective (KSS) scores was very strong, evidence for systematic errors in both the mean and the calibration was also found. Correction of these errors resulted in strengthening of correlation (r = 0.99) and establishing one-to-one correspondence between the steps of objective and subjective scales. The results indicate that scores from 5 to 0 on the objective alertness scale can be anchored to minimal, mild, moderate, marked, severe, and disabling levels of sleepiness.