A sleep physiologist's view of the drowsy driver

Drowsy driving is dangerous because of the impairment of driving skills that it causes. Unfortunately, the conceptual basis that underlies much of the multi-disciplinary research on this topic is muddled. The same poorly defined terms, such as fatigue and sleepiness, are used differently by different disciplines and researchers. Some new definitions and concepts are proposed here which may be helpful, as least as a stimulus for discussion by others. Drowsiness, sleepiness and fatigue are distinguished. A new conceptual model of sleepiness is outlined, based on a mutually inhibitory interaction between a putative sleep drive and a wake drive. Sleepiness, defined as sleep propensity, is a function of the relative strengths, not the absolute strengths, of the sleep and wake drives. The measurement of sleepiness requires some new variables such as instantaneous sleep propensity, to be distinguished from either the situational or the average sleep propensity. A subject's instantaneous sleep propensity depends on many variables including his average sleep propensity in daily life, the time of day, the duration of prior wakefulness, the subject's posture, physical and mental activity at the time, and individual differences based on psychophysiological traits. The relationship between dozing at the wheel while driving and crashing the vehicle may not be as straightforward as it appears at first.