The effects of asking for verbal ratings of sleepiness on sleepiness and its masking effects on performance q Kosuke Kaida a,b,c, * , Torbjo ¨rn A ˚ kerstedt b,c , Go ¨ ran Kecklund b,c , Jens P. Nilsson c , John Axelsson b,c a National Institute of Occupational Safety and Health (JNIOSH), 6-21-1, Nagao, Tamaku, Kawasaki, Kanagawa 214-8585, Japan b Department of Public Health Sciences, Karolinska Institutet, Stockholm, Sweden c National Institute for Psychosocial Medicine (IPM), Stockholm, Sweden Accepted 8 March 2007 Available online 26 April 2007 Abstract Objective: This study investigated whether verbal rating sleepiness will itself affect sleepiness and performance. Methods: Thirteen healthy male volunteers (mean age, 26.9 years) performed two 40-min vigilance tests, one of which involved verbal ratings every 4 min using the Karolinska sleepiness scale and another of which did not involve any ratings during the test. Results: Repeated rating of sleepiness significantly reduced post-test sleepiness and improved the subjective perception of performance, and also reduced alpha power density (i.e., a physiological indicator of sleepiness). However, performance was not improved by the ratings. Conclusions: The act of rating affects subjective and EEG measures of sleepiness. Presumably this occurs through the modest stimulation involved in this act. Significance: Methodologically one should be aware of subtle effects of the rating situation on sleepiness. From a practical point of view, it would be important for safety management since subjective sleepiness and performance are easily dissociated, which might interfere with risk perception. Ó 2007 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. Keywords: Verbal rating of sleepiness; Masking effect; KSS; EEG; Eye-blink; Heart rate variability 1. Introduction Repeated verbal ratings are often used to describe tem- porary subjective sleepiness patterns in laboratory and field studies. Laboratory studies show that sleepiness is strongly influenced by sleep duration (Jewett et al., 1999), time awake, and circadian-mediated low temperatures (Dijk and Czeisler, 1995). Workers on a night shift show increased subjective sleepiness (Lowden et al., 2004). In addition, laboratory experiments have shown that elapsed time on a task increases subjective sleepiness (Dinges et al., 1997; Horne and Baulk, 2004). The context of the rating situation also influences sleepiness ratings (Dinges, 1995; Yang et al., 2004); for example, airplane pilots’ sleep- iness decreases during take-off and landing (Eriksen et al., 2006). According to recent evidence, sleepiness occurs as an interaction between two independent mechanisms, that is, a ‘‘sleep/homeostatic drive’’ and a ‘‘wake drive’’ (Bonnet and Arand, 2001; Cluydts et al., 2002; De Valck and Clu- ydts, 2003). These two drives are often described as the ability to fall asleep and the ability to stay awake, and can be measured by the multiple sleep latency test (MSLT; Carskadon and Dement, 1982) and the maintenance of wakefulness test (MWT; Mitler and Miller, 1996; Cluydts 1388-2457/$32.00 Ó 2007 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.clinph.2007.03.004 q This work was performed at the National Institute for Psychosocial Medicine (IPM), Stockholm, Sweden. * Corresponding author. Address: National Institute of Occupational Safety and Health (JNIOSH), 6-21-1, Nagao, Tamaku, Kawasaki, Kanagawa 214-8585, Japan. Tel.: +81 44 865 6111; fax: +81 44 865 6124. E-mail address: kaida-kosuke@umin.ac.jp (K. Kaida). www.elsevier.com/locate/clinph Clinical Neurophysiology 118 (2007) 1324–1331