Cognitive effects of sleepiness

Introduction There is a well-documented, inverted U-shaped relationship between arousal and cognitive performance, which is known as the Yerkes–Dodson law (see Figure 7.1). According to this law, optimal performance requires a specific level of arousal, and either too much or too little arousal degrades performance. Thus, reduced arousal, such as that associated with sleepiness, results in cognitive performance deficits. Indeed, sleepiness induced by sleep loss and/or circadian rhythm has been demonstrated to result in cognitive impairment in many studies (see Figure 7.2). In the Yerkes–Dodson paradigm, the optimal level of arousal varies between different performance tasks, depending on task difficulty, prior experience with the task, and other factors. Similarly, the effect of sleepiness seems to vary from one task to another, but there is uncertainty about the determinants of this inter-task variability. Reasons for this uncertainty include a range of methodological confounds in studies of sleep loss and circadian rhythm, the existence of large individual differences in the expression of sleepiness, and a shortage of studies systematically investigating inter-task differences in the context of sleepiness. This shortage may be attributed, in part, to two trends in the literature about sleepiness due to sleep deprivation and sleep disorders. First, there has been considerable emphasis on achieving optimal sensitivity of cognitive performance tasks for detecting and quantifying sleepiness.