The investigation included six drivers engaged in day driving and six drivers engaged in night driving. Changes in wakefulness were analysed by means of a questionnaire where the drivers were asked to rate their wakefulness on a 100 mm rating scale. Changes in wakefulness were analysed during intake of food with higher and lower contents of fat. The day-drivers had their intakes as breakfast and lunch, the night-drivers as dinner and between meals. No significant difference was observed between the two types of intake, meaning that the balance between fat, protein and carbohydrate does not effect the development of drowsiness.