Relation of circadian profile on driving performance

Aim: The major determinants of fatigue of the truck drivers have been identified in the duration of wakefulness, inadequate sleep and sleep disorders, extended working shifts and time of the day. It has been also suggested that interindividual differences in the spontaneous sleep-awake cycle may influence, at least at certain times of the day, the levels of performance; thus the chronotype of the drivers (morning or evening type) might be related to individual differences of alertness during the time of the day.The aim of the present study is to evaluate the effects of psychophysiological factors such as age, personality traits, behavioural factors and urinary melatonin, taken as marker of circadian rhythm, on vigilance levels of professional drivers.Methods: The study was carried out on male truck drivers aged 34-53 years .On working days, before and at the end of driving shifts the drivers were submitted to the following performance measures: Vienna Reaction Test (RT) and Vienna Deterrnination Test (DT), provided by Schuhfried (Austria).6-sulfatoxymelatonin urinary levels were measured in two urine specimens: a first morning urine specimen and a specimen of the urine excreted, over a period of about 4 hours, before the end of the driving shift. Personality traits were evaluated by the Italian version of the Big Five Questionnaire (BFQ). Records of road and traffic conditions (motorways, suburban and urban roads, traffic jams, visibility), distance travelled and length of each driving shift were also collected.Results: A significant improvement of reactive stress tolerance (reaction times and number of correct reactions by Vienna DT) has been observed at the end of the driving shift compared to measures made before starting, while no significant variation was observed in Vienna RT results.The age of drivers resulted significantly related with motor times measured by Vienna RT both before (r = 0.358; P = 0.025) and at the end (r= 0.416; p= 0.008) of the driving period.It was observed a significant relationship between reaction time measured by Vienna RT and urinary melatonin levels in urine collected before the driving shift.Discussion: The negative relationship between the measures of drivers performance and the melatonin levels in the ear1y morning observed in our study and the wide interindividual variability of this marker of circadian rhythm suggest the opportunity of scheduling the driving shift taking into account the personal differences of endogenous circadian profile. For what concern the role of personality factors it was also observed that emotional stability significantly impair the driving performance.