An experimental study of the acute effects of meals, noise and nightwork showed that there was a post-meal impairment in detection of targets in a cognitive vigilance task. This was found both during the day and at night, although certain features of the results suggested that the day and night effects were not equivalent. Noise increased the number of false alarms but reduced the post-meal impairment in hit rate. Subjects with low levels of trait or state anxiety showed the greatest post-lunch impairments in performance, but this effect was reduced when the meal was eaten at night.