2006 Volume 9 Issue 1 Pages 7-12
In order to study the circadian rhythm of cognitive function related to auditory frequency system and melatonin secretion for people who preference to be active in the morning or at night, we have done an experiment at three time sessions. On the basis of a morningness/eveningness questionnaire, nine morning-type subjects (M-types) and ten evening-types subjects (E-types) were selected to participate in the present experiment. Diurnal changes of human cognitive function under 250/500 Hz and 1000/2000 Hz conditions were assessed hourly by auditory event-related brain potential using an oddball task. The melatonin rhythm was estimated by cosine-fitting curve of saliva melatonin concentration. Saliva samples (3–5 ml) were collected every hourly to determine peak salivary melatonin concentration before the measurement of P300. A marginally significant main effect of time of day (P=0.0512) and a significant main effect of stimulus frequency (P<0.05) in reaction time (RT) were found. There was a significant interaction between the circadian typology and time of day in RT (P<0.001). A significant main effect of circadian typology (P<0.001) and time of day (P<0.05) in P300 amplitude was found. The P300 amplitude between M-types and E-types was significantly different at 10:00, 11:00, 12:00, 21:00, 23:00, 24:00 and 01:00. A significant main effect of stimulus frequency in P300 amplitude was found (P<0.01). There was a significant interaction among the circadian typology, time of day and stimulus frequency in P300 amplitude (P<0.05). The P300 amplitude obtained after the low-frequency stimulus was significantly different than that obtained after the high-frequency stimulus at 10:00, 12:00, 19:00, 20:00, 23:00 and 24:00 for M-types, and at 12:00, 19:00, 21:00 and 01:00 for E-types (P<0.05). These findings suggest that the diurnal change of human cognitive function was related to different circadian typology, and may have important implications for industries employing different work shifts or workers using flexible working hours.
