There has been extensive research on the behavioural effects of caffeine, with most studies being conducted in the laboratory. Epidemiological studies have examined the effects of regular consumption patterns, but often the accuracy of measurement of caffeine consumption is low. There is a need to examine the effects of caffeine consumption in free-living participants, and that was the aim of the present study.

Twenty three participants completed a cross-over study where they were provided with decaffeinated coffee and tea on one day and caffeinated products on the other. The caffeine manipulation was double-blind. After consuming their normal breakfast, the participants visited the laboratory at 08.00 and carried out alertness ratings and a simple reaction time task. They also collected the coffee and tea at this time. Participants recorded their consumption of the coffee and tea and returned to the laboratory at 17.00.

The alertness and reaction times scores were transformed to per cent change from baseline. On the day consuming caffeinated products, the percentage increase in alertness was significantly higher than on the day consuming decaffeinated products. On the decaffeinated day, volunteers had a greater increase in simple reaction time than on the caffeine day. The beneficial effect of caffeine was greater in the high consumers. Alertness at the start of the day predicted subsequent consumption of caffeinated beverages, with lower alertness at that time being associated with greater subsequent consumption.

These results demonstrate that when individuals are free to consume as many caffeinated or decaffeinated cups of coffee and tea, the caffeinated products lead to higher alertness and faster simple reaction time in the late afternoon. Those who consumed higher amounts of caffeine showed greater benefits than the low consumers. Lower alertness in the early morning led to greater consumption of caffeinated beverages. Overall, these results confirm that consumption of caffeine reduces the build-up of fatigue over the day, which has strong implications for safety-critical activities.