CIRCADIAN-RHYTHMS IN HUMAN-PERFORMANCE AND MOOD UNDER CONSTANT CONDITIONS

This study explored the relationship between circadian performance rhy thms and rhythms in rectal temperature, plasma cortisol, plasma melato nin, subjective alertness and well-being. Seventeen healthy young adul ts were studied under 36 h of 'unmasking' conditions (constant wakeful bedrest, temporal isolation, homogenized 'meals') during which rectal temperatures were measured every minute, and plasma cortisol and plas ma melatonin measured every 20 min. Hourly subjective ratings of globa l vigour (alertness) and affect (well-being) were obtained followed by one of two performance batteries. On odd-numbered hours performance ( speed and accuracy) of serial search, verbal reasoning and manual dext erity tasks was assessed. On even-numbered hours, performance (% hits, response speed) was measured at a 25-30 min visual vigilance task. Pe rformance of all tasks (except search accuracy) showed a significant t ime of day variation usually with a nocturnal through close to the tro ugh in rectal temperature. Performance rhythms appeared not to reliabl y differ with working memory load. Within subjects, predominantly posi tive correlations emerged between good performance and higher temperat ures and better subjective alertness; predominantly negative correlati ons between good performance and higher plasma levels of cortisol and melatonin. Temperature and cortisol rhythms correlated with slightly m ore performance measures (5/7) than did melatonin rhythms (4/7). Globa l vigour correlated about as well with performance (5/7) as did temper ature, and considerably better than global affect (1/7). In conclusion : (1) between-task heterogeneity in circadian performance rhythms appe ared to be absent when the sleep/wake cycle was suspended; (2) tempera ture (positively), cortisol and melatonin (negatively) appeared equall y good as circadian correlates of performance, and (3) subjective aler tness correlated with performance rhythms as well as (but not better t han) body temperature, suggesting that performance rhythms were not di rectly mediated by rhythms in subjective alertness.