Interactions between inter-individual and inter-task differences in the diurnal variation of human performance.

Thirty-six undergraduate subjects were divided into 'larks' and 'owls' by median split using the morningness test devised by Horne and Ostberg. Each was given a battery of two performance tests at six times of day, in a design that counter-balanced for practice effects. Oral temperatures were also recorded. Slight (1 hr) phase differences (in the predicted direction) were observed between 'larks' and 'owls' in temperature rhythm, and in the performance test that paralleled temperature (a serial search task). Much more striking differences, though, were observed in the cognitive (logical reasoning) performance test which (as predicted) peaked earlier in the day, on average, than the serial search task. Not only did 'larks' peak much earlier in the day than 'owls', but there were also quite dramatic differences between the two groups in amplitude and shape. Possible mechanisms regarding oscillatory control and performance strategy are discussed.