Caffeine improved performance best at the 7am testing time - when the body's 24 hour rhythm of arousal is at its lowest. By contrast Mitchell found that the high-caffeine user group didn't perform as well as the others on more complex tasks such as verbal reasoning. The evening people also scored higher on both the extroversion scale and the impulsivity, risk-taking scale. They typically find it difficult to get up in the morning and give themselves big hits of caffeine to wake themselves up. Dr Mitchell also says there may be a link between extrovert behaviour and higher caffeine usage.
Caffeine seems to disrupt the body clock rhythms of extreme morning people.
By contast the rhythms of the evening types are disturbed by the absence of caffeine. Dr Mitchell believes evening people use caffeine to keep their rhythms regular. One of the strongest findings was that if morning people had caffeine after about 10am, their mood went down quite dramatically. This could be because caffeine has disrupted their underlying rhythms.