Driving drowsy also worsens driver distraction.

Objectives: Laboratory-based studies show that drowsiness increases the propensity to become distracted. As this phenomenon has not been investigated in drowsy drivers, we underwent a pilot study under realistic monotonous driving conditions to see if distraction was more apparent when drowsy; if so, how does it affect driving performance?

Methods: A repeated measures counterbalanced design whereby participants drove for two hours in a fully interactive car simulator during the bi circadian afternoon drive, after a night of either normal (baseline) or restricted sleep to five hours (sleep restriction). Videos of drivers' faces were analysed blind for short (<3 s) and long (>3 s) distractions, in which drivers took their eyes off the road ahead. These results were compared with the likelihood of simultaneous lane-drifting incidents, when at least two wheels left the driving lane.

Results: More distractions occurred after restricted sleep (p<0.005) for both short and long distractions (p<0.05). There was an overall significant (p<0.02) positive correlation between distractions and driving incidents for both conditions but with significantly more distraction-related incidents after sleep restriction (p<0.03).

Conclusions: Following restricted sleep, drivers had an increased propensity to become distracted, which was associated with an increased likelihood of poor driving performance as evidenced by the car leaving the driving lane.