Abstract

Individual differences in the phase of circadian (around 24 h) rhythms are thought to be important in determining adjustment to shift work and rapid time-zone transitions. Attempts to predict such phase differences on the basis of paper and pencil ‘personality’ tests have concentrated on extraversion and morningness, of which Kerkhof (1985), in a recent review of this literature, concluded morningness was the more important. However, the literature on which this conclusion was based suffers from a number of problems. The present study attempted to overcome these problems by examining the trends over a complete 24 h cycle for a range of performance and psychophysiological measures in students with extreme scores for both extraversion and morningness. In general, the results support KerkhoFs conclusion. However, reliable phase differences associated with morningness were confined to subjective ratings of alertness, oral temperature, and, in combination with extraversion, choice reaction time. Two alternative interpretations of this pattern of results are considered, and their practical implications discussed.