Effects on vigilance performance and sleepiness of alcohol given in the early afternoon (‘post lunch’) vs. early evening

Abstract

There is a propensity for sleepiness in the early afternoon ('post-lunch dip'), that contrasts with the circadian peak of alertness in the evening. Alcohol has a soporific effect, and one might expect it to be more potent in the early afternoon than in the evening. This is a little explored area. Four, two or nil (English) units of alcohol (producing maximum blood alcohol concentrations [BACs] of approx. 70, 35 and less than 5 mg alcohol 100/ml blood respectively) were given with food to eight women, at 1300h or 1830h, in a double-blind randomized repeated-measures design. After this consumption, subjects underwent 1 h of vigilance testing. They rated themselves for sleepiness on a half-hourly basis. There were significant main effects for dose and for time of day with: reaction time, hits and sleepiness scores. There was also a significant dose x time of day interaction with: reaction time and hits. That is, two and four units of alcohol produced greater impairment in the afternoon compared with the corresponding evening values. Implications of these findings for driving behaviour are discussed (the maximum legal BAC for car drivers in the UK is 80 mg/100 ml blood).