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28 120 100 Average PVT Lapses in Last 24 Hours of Second 36-Hour Sleep Deprivation 80 60 40 20 0 0 20 40 60 80 100 120 Average PVT Lapses in Last 24 Hours of First 36-Hour Sleep Deprivation Figure 13. Scatter plot showing large variations in vigilance (alertness) among 18 sleep deprivation subjects, but high similarities between individuals' performance during the first and second sleep deprivation periods. (SOURCE: Van Dongen et al. 2004) scores for the 18 subjects. Only one of the 18 subjects per- ceptibility to drowsiness. This was illustrated by the FHWA- formed substantially differently across the two sessions, and sponsored DFAS and by a study of fatigue associated with the many subjects performed almost identically. use of sleeper berths. Studies have shown that humans are gen- A factor analysis of scores on the 13 neurobiological mea- erally not very good judges of their own levels of sleepiness, but sures revealed three common personal factors underlying the there are even large individual differences in the accuracy of score differences: self-evaluation (sleepiness and mood), self-assessment. Variations of amount of nightly sleep are one cognitive processing (ability to engage in complex thinking), obvious source of individual differences in alertness, but sig- and vigilance (behavioral alertness). Intuitively, one would nificant differences are seen even when subjects receive con- predict that hours of sleep deprivation would be a stronger trolled amounts of sleep. Moreover, a person's ability to stay predictor of subject performance than individual differences, awake and perform during sleep deprivation seems to be but this was not the case. On every measure, the influence of remarkably consistent over time, even though there are large individual differences was stronger than the influence of sleep differences among different people. deprivation duration. The authors summarized their study findings as follows: 4.7 PERSONALITY In this study involving repeated exposure to sleep deprivation under carefully controlled laboratory conditions, we found In the present context, "personality" refers to enduring per- that neurobehavioral impairment from sleep loss was sig- sonal traits or tendencies that affect behavior. Personality is nificantly different among individuals, stable within indi- most often viewed in relation to interpersonal interaction viduals, and robust relative to experimental manipulation of (e.g., introversion-extroversion), but personality traits can sleep history. Thus, this study is the first to demonstrate that inter-individual differences in neurobehavioral deficits from also be associated with driving and other safety-related sleep loss constitute a differential vulnerability trait (Van behaviors. In the survey, personality traits such as aggres- Dongen et al. 2004). siveness, impulsivity, and inattentiveness were rated by both respondent groups as having the highest associations with In summary, it appears that there are significant individual risk of the various factors listed, which included demographic, differences among commercial drivers in the incidence and sus- experience, personal/family, and medical factors. Corsi and