Mild hypohydration increases the frequency of driver errors during a prolonged, monotonous driving task

•Mild hypohydration has been shown to cause impaired cognitive function and altered mood.•This study reports an increase in driver errors with mild dehydration.•Error incidence increased over time, but occurred at a greater rate following fluid restriction•Higher subjective feelings of thirst, as well as impaired concentration and alertness were also apparent•Driver education programmes should also encourage appropriate hydration practices.

The aim of the present study was to examine the effect of mild hypohydration on performance during a prolonged, monotonous driving task.MethodsEleven healthy males (age 22 ± 4 y) were instructed to consume a volume of fluid in line with published guidelines (HYD trial) or 25% of this intake (FR trial) in a crossover manner. Participants came to the laboratory the following morning after an overnight fast. One hour following a standard breakfast, a 120 min driving simulation task began. Driver errors, including instances of lane drifting or late breaking, EEG and heart rate were recorded throughout the driving taskResultsPre-trial body mass (P = 0.692), urine osmolality (P = 0.838) and serum osmolality (P = 0.574) were the same on both trials. FR resulted in a 1.1 ± 0.7% reduction in body mass, compared to − 0.1 ± 0.6% in the HYD trial (P = 0.002). Urine and serum osmolality were both increased following FR (P < 0.05). There was a progressive increase in the total number of driver errors observed during both the HYD and FR trials, but significantly more incidents were recorded throughout the FR trial (HYD 47 ± 44, FR 101 ± 84; ES = 0.81; P = 0.006)ConclusionsThe results of the present study suggest that mild hypohydration, produced a significant increase in minor driving errors during a prolonged, monotonous drive, compared to that observed while performing the same task in a hydrated condition. The magnitude of decrement reported, was similar to that observed following the ingestion of an alcoholic beverage resulting in a blood alcohol content of approximately 0.08% (the current UK legal driving limit), or while sleep deprived.