Do night owls drive more recklessly?

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By Dr. Simon Kyle

It is well known that traffic accidents follow a circadian rhythm; accidents tend to peak during dips in alertness, typically in the early afternoon and in the early hours of the morning. However, it is also true that some people are more “morning types” whereas others would consider themselves “evening types”, and that this so-called chronotype may have an impact on patterns of driving performance. Cognitive performance is optimal when assessment takes place during peaks in arousal, governed by internal biological time regardless of clock time per se, and hence assessments out with this peak may be associated with relatively poorer performance.

To test this hypothesis in relation to driving, in a new study published in Chronobiology International, participants were recruited according to chronotype (n=19 morning types; n=28 evening types). Participants were assessed at two time points (in the morning and evening) on a Honda motor bike riding simulator. Participants were assessed in counterbalanced order both at their optimal time of day and outside this optimal time of day. Assessment took place at regular commuting times in Spain i.e. 9-11am and 6-8pm. The simulator involved projection of a driving scenario on the screen in front of the participant and their task was to negotiate difficult driving situations (e.g. obstacles designed to lead to a crash if the driver did not react appropriately).

The main findings were that evening participants demonstrated reduced “safe behaviors” (e.g. avoiding hazards without pressing hard on brakes, following the speed limit) during the morning session relative to those with a morning chronotype. Moreover a similar pattern was found for “precaution behaviors” and self-reported alertness at time of testing. No significant effects were observed for the evening session; performance was similar across groups. Prior nights’ sleep quality did not moderate the effects. There was no chronotype effect in relation to number of “accidents”.

The authors point to evidence indicating that rhythms in body temperature and morning cortisol rise, both associated with alertness, tend to peak earlier in those with a morning preference and hence may account for the chronotype differences (better morning performance in “morning types”). They conclude:

“In light of our results, chronotype, along with driving schedule, should be taken into account in theoretical models of driving behavior. Importantly, these variables might be crucial for the prevention of occupational risks in driving-related jobs, therefore their incorporation in hours-of-service regulations is advised.”

Del Rio-Bermudez, C., Diaz-Pedra, C., Catena, A., Buela-Casal, G., Di Stasi, L.L. (2014). Chronotype-dependent circadian rhythmicity of driving safety. Chronobiology International, doi:10.3109/07420528.2013.876427

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