Validation of the Karolinska sleepiness scale against performance and EEG variables

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Abstract

Objective

The Karolinska sleepiness scale (KSS) is frequently used for evaluating subjective sleepiness. The main aim of the present study was to investigate the validity and reliability of the KSS with electroencephalographic, behavioral and other subjective indicators of sleepiness.

Methods

Participants were 16 healthy females aged 33–43 (38.1±2.68) years. The experiment involved 8 measurement sessions per day for 3 consecutive days. Each session contained the psychomotor vigilance task (PVT), the Karolinska drowsiness test (KDT—EEG alpha & theta power), the alpha attenuation test (AAT—alpha power ratio open/closed eyes) and the KSS.

Results

Median reaction time, number of lapses, alpha and theta power density and the alpha attenuation coefficients (AAC) showed highly significant increase with increasing KSS. The same variables were also significantly correlated with KSS, with a mean value for lapses (r=0.56).

Conclusions

The KSS was closely related to EEG and behavioral variables, indicating a high validity in measuring sleepiness.

Significance

KSS ratings may be a useful proxy for EEG or behavioral indicators of sleepiness.

Introduction

Sleepiness is involved in a large part of the accidents in transportation and in other areas of industry (Maycock, 1996). Subjective reports are a convenient way of gathering information about sleepiness in field and laboratory studies. For reports of habitual sleepiness, the Epworth sleepiness scale (Johns, 1991) is frequently used. For reports of instantaneous sleepiness (across the day and night), visual analogue scales (Monk, 1989) or Likert scales, like the 7-graded Stanford sleepiness scale (Hoddes et al., 1973) or the 9-graded Karolinska sleepiness scale (KSS) (Åkerstedt and Gillberg, 1990), are often used.

The KSS was originally developed to constitute a one-dimensional scale of sleepiness and was validated against alpha and theta electroencephalographic (EEG) activity as well as slow eye movement electrooculographic (EOG) activity (Åkerstedt and Gillberg, 1990). It has been widely used and provided reasonable results in studies of shift work (Axelsson et al., 2004, Gillberg, 1998, Härmä et al., 2002, Ingre et al., 2004, Sallinen et al., 2004, Sallinen et al., 2005), jet lag (Suhner et al., 1998), driving abilities (Åkerstedt et al., 2005, Belz et al., 2004, Horne and Baulk, 2004, Kecklund and Åkerstedt, 1993, Otmani et al., 2005, Philip et al., 2005, Reyner and Horne, 1998), attention and performance (Gillberg et al., 1994, Gillberg et al., 1996, Kräuchi et al., 2004, Reyner and Horne, 1998) and clinical settings (Schwartz, 2005, Söderström et al., 2004).

In terms of validation, there have been several studies showing relatively strong positive intra-individual correlations between the KSS and alpha and theta EEG activity (Åkerstedt and Gillberg, 1990, Horne and Baulk, 2004). Reyner and Horne (1998) also demonstrated that falling asleep at the wheel in a driving simulator was always preceded by increased KSS score. While these results indicate a relatively good intra-individual relationship between the KSS and electrophysiological and behavioral variables, we know rather little about the ‘meaning’ of different levels of the KSS in terms of electrophysiology or behavior. This concerns the characteristics of electrophysiology or as well as behavior at different levels of the KSS, giving an impression of the shape of the relationship.

The only study looking at the characteristics of electrophysiology at different levels of the KSS used the Karolinska drowsiness test (KDT) (Åkerstedt and Gillberg, 1990) to evaluate electrophysiological sleepiness. This test is based on the power density of the EEG during ‘eyes-open’ or ‘eyes-closed’. In the study mentioned, alpha and theta power density increased with sleepiness in the eyes-open condition, whereas alpha power density decreased and theta power density increased during the eyes-closed condition. During ‘eyes-open’ in normal alertness, brain activities are dominated by waves within the beta band (>13 Hz). With increased drowsiness in ‘eyes-open’, the proportion of alpha and theta activity is increased. During ‘eyes-closed’ and alertness, brain activity is dominated by alpha activity (8.0–12.0 Hz) which is replaced by theta activity with increasing sleepiness.

Another EEG-based approach for sleepiness is alpha attenuation test (AAT) (Stampi et al., 1995). In the AAT, the feature of alpha activity during normal alertness is used. The alpha activity tends to decrease as an ‘eyes-closed’ participant gets sleepier. On the other hand, in an ‘eyes-open’ individual, the alpha activity normally increases as a function of increased sleepiness. Usage of the ratio between ‘eyes-closed’ and ‘eyes-open’ alpha activity would discriminate a difference between levels of sleepiness and minimize inter-individual variability in alpha activity.

It would be also important to describe commonly used performance variables at different levels of the KSS. One such measure is the psychomotor vigilance task (PVT), which seems sensitive to sleep loss (Dinges et al., 1997). Although the PVT is frequently used in sleepiness or sleep deprivation studies, the correlation between the PVT performances and EEG parameters has not ever been reported (Drummond et al., 2005).

Another interesting point is that all previous studies have, for natural reasons, involved both high and low levels of sleep loss. It seems a reasonable assumption that intra-individual correlations would be stronger if sleep loss and/or the circadian trough are included in the study since this would likely increase the intra-individual variation. Thus, it is an interesting question whether measurements made under conditions of normal night sleep would provide a reasonable covariation between the KSS and electrophysiological and behavioral variables.

The purpose of the present study was to investigate the relation between the KSS and electrophysiological and behavioral measures of sleepiness under conditions of several days and with relatively normal night sleep. In the original validation study (Åkerstedt and Gillberg, 1990), only 7 measurements were made per individual, thus limiting the stability of the computed correlations. Clearly, there is a need for an increased number of measurements, either with a higher density than the 4-hourly ones in the previous study or through inclusion of several days of measurements. The variables chosen for validation was the alpha attenuation test (Stampi et al., 1995), the psychomotor vigilance task (Dinges and Powell, 1985), and a visual analogue scale for sleepiness (Monk, 1989). In addition, the Karolinska drowsiness test (Åkerstedt and Gillberg, 1990) should be included for comparison with the previous study.

The main focus was on the form of the relation between the KSS and the other variables, but also intra-individual correlations were computed. Other results from the present study have been presented in the form of effects of light treatment on sleepiness (Kaida et al., 2006).

Section snippets

Participants and design

Participants were 16 healthy female paid volunteers aged 33–43 (38.1±2.68) year. It was extremely hard to find a male participant who met with the selection criteria, so only females were selected for the present study. All participants met the following criteria: (1) a normal sleep-wake cycle classified as ‘intermediate type’ according to the Morningness–Eveningness questionnaire (Horne and Ostberg, 1976, Ishihara et al., 1986), (2) no report of any physical or mental health problems, and a

Results

Total sleep/rest time measured by the actiwatch prior to the experimental days was 362.2 (±52.56) min for day 1, 365.3 (±49.17) min for day 2, 359.8 (±53.86) min for day 3, respectively. The variation across days was not significant [F(2, 30)=1.09, P=0.92, ε=0.92].

Mean values, standard deviations (SD), and the range of the original data are shown in Table 1.

In order to study the form of the relation between the KSS-J and the other parameters, the KSS-J was divided into bins (1–3, 4–5, 6, 7 and

Discussion

All variables except for theta power density with eyes-closed showed clear relations to the KSS-J despite the fact that the range of variation of sleepiness probably was lower than it would have been if sleep deprivation had been involved. It is notable that the correlations between EEG and PVT performances would be the first report to our best knowledge.

The results are similar to a number of other studies showing relatively high correlations between performance measures and subjective

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