Circadian mismatch and cardiovascular response to a performance challenge: Larks in morning and evening work sessions

We presented morning chronotype ("Lark") university undergraduate volunteers a more or less difficult Sternberg-type recognition memory task either in the morning (8-11 am) or in the evening (5-8 pm) with instructions that they could win a prize if they were 85% successful. We established morning chronotype using the Composite Scale for Morningness (Smith et al., 1989), employing a tertile split on a pool of scale scores that ranged from 13 (extreme eveningness) to 55 (extreme morningness). Participants had scores above 37, with most participants identifying as White/Caucasian, Hispanic/Latino, or Black/African-American. Among women (final sample n = 81), systolic blood pressure and mean arterial pressure responses assessed during work formed a crossover pattern, being positively correspondent to difficulty in the morning but negatively correspondent to difficulty in the evening. Heart rate and heart pre-ejection period responses ran parallel in the morning but not the evening. Among men (final sample n = 41), cardiovascular responses differed neither with difficulty nor with time. Findings for women support the extension of a recent analysis of fatigue influence on effort and associated cardiovascular responses to the phenomenon of circadian mismatch. Findings for men do not support the extension but should be interpreted guardedly in light of prohibitively low cell ns and unexpected findings on key subjective measures.