Subjective sleepiness: an undervalued early sign of insufficient sleep?

Abstract

How do we know whether our sleep is sufficient? How do we know when to stop driving because we are too sleepy to drive safely? There are no easy answers to these frequently asked questions, and the sleep research and sleep medicine community have made considerable efforts to develop methodology to assess impaired brain functioning associated with insufficient sleep; but what are the first signs of insufficient sleep? In the current issue, Akerstedt and colleagues review their work on the Karolinska Sleepiness Scale (KSS) (Akerstedt et al., 2014). Obviously, the basic assumption when using a subjective self-assessment tool is that ‘the brain knows’ when it does not get enough sleep. This seems to be a reasonable assumption. In fact, it may be argued that a ‘clever brain’ would develop mechanisms that can identify insufficient sleep before brain function is impaired. Are our cars not equipped with indicators to tell us to go to the nearest petrol station before it is too late, and are most maintenance schemes not scheduled in such a way as to prevent malfunctioning? Sleepiness and the associated (sometimes overwhelming) tendency to fall asleep may be the brain's way of signalling insufficient sleep and that it is time for some preventive maintenance. Within this context, the main danger associated with sleepiness is the process of falling asleep and the associated disengagement with the environment. Within this framework, an assessment of sleepiness that can detect the very first signs of the process becomes central. The multiple sleep latency test (MSLT) is a gold standard for assessing objective sleep tendency: how long does it take to fall asleep? Obviously the MSLT is not implemented easily in everyday situations. Other objective measures of acute sleepiness rely on behavioural signs associated with falling asleep, such as eye closure, but at this point the tank is already nearly empty. The KSS is a simple and easy-to-implement self-assessment tool and the review highlights its ability to detect sleepiness in a wide variety of situations. The sensitivity of the KSS is considerable, and the associations between higher scores on the KSS and impaired performance on relevant tasks, such as driving, are impressive. Of course, this is not to say that the KSS will detect sleepiness, or the consequences of insufficient sleep, in every situation and in every individual, but this is probably true for every ‘sleepiness’ assessment tool. To have a standard sleepiness assessment tool which can be used in a wide range of field, laboratory and clinical settings will greatly facilitate comparisons across laboratories, studies and populations. Continued comparison of the KSS to objective measurements of sleepiness, performance and impaired attention may lead ultimately to increased awareness among all of us to pay attention to the first signs of insufficient sleep. This, in turn, may help us to change our sleep schedules and avoid situations in which sleepiness can kill. Other papers in the current issue explore the importance of sufficient sleep for memory consolidation in children (Ashworth et al., 2014), highlight similarities between the electroencephalography (EEG) during epilepsy and sleep (Gast et al., 2014) and describe how EEG slow wave activity during development reflects both sleep regulatory processes and developmental changes in the brain (Olini and Huber, 2014).