An experiment was carried out to examine the effects of coffee on performance and alertness in the day and at night. The results showed that caffeinated coffee had a beneficial effect on alertness and improved performance on a variety of tasks in both day and night sessions. The effects were often very large. For example, at night, consumption of caffeinated coffee produced comparable alertness ratings to the day-time ratings given when juice was drunk. In contrast to the effects of caffeinated coffee, the difference between the decaffeinated coffee and juice were small and variable. Overall, these results clearly demonstrate the beneficial effects of consuming caffeinated coffee, and show that this effect is comparable in the day and night.