'Post-lunch' sleepiness during prolonged, monotonous driving - effects of meal size.

The 'post-lunch' dip is a bi-circadian phenomenon, largely unrelated to lunch, and worsened by a disturbed prior night's sleep. Despite anecdotal claims of adverse effects of larger lunches on afternoon driving ability, there is little actual driving data to support this belief. Although there have been various (non-driving) laboratory studies assessing meal size and micronutrient effects on psychological performance tests, findings are mixed. Moreover, most have not utilised heightened afternoon sleepiness after a shortened night's sleep, and few tested beyond 20 min. Using a real car interactive simulator having full size screen projection, we compared the effects on a 2h monotonous afternoon drive, of two very similar, palatable lunches ('light': 305 cal vs 'heavy': 922 cal [having 3× fat and 2× carbohydrate contents]), given double blind in a repeated measures counterbalanced design, to 12 young male drivers whose prior night's sleep had been restricted to 5h. Sleepiness-related lane drifting ('incidents'), subjective sleepiness and EEG (4-11 Hz power - indicative of sleepiness) were logged throughout. The heavy lunch caused significant increases to both incidents and EEG power, and a trend for greater subjective sleepiness. All three indices showed a significant worsening of sleepiness over the drive under both lunch conditions. Whilst there were no significant condition×time interactions, there was no difference between lunches for at least the first 30 min of the drive when, thereafter, the differences appeared. Ours was a realistic driving study, utilising typical lunches, following an unexceptional level of prior sleep loss, and where a heavy lunch exacerbated inherent sleepiness, to further impair monotonous driving.