Diurnal variation of salivary cortisol and amylase activity in fatigued state

Abstract

Saliva has great potential as a bio-fluid for point-of-use measurement of the human stress response. The purpose of this study was to evaluate the diurnal variation of salivary cortisol (CORT) and salivary amylase activity (SAA) under the condition of a fatigued state. The subjects were 11 emergency medical doctors who worked for the Department of Emergency and Critical Care Medicine in 24 h-shifts, and their diurnal variations were measured three times in one month during the 24h-shift workday. As a control experiment, 6 subjects’ diurnal variations were measured during a holiday period. The saliva samples were collected 4 times in total during an examination. The mean CORT of a workday showed its lowest value of 1.07 ± 0.55 ng/ml before sleep (24:00), and was highest at 4.76 ± 3.16 ng/ml at the start of a daytime duty (9:00) (p < 0.01). In contrast, the mean CORT of a holiday showed its lowest value of 1.48 ± 1.00 ng/ml in the afternoon and a highest value of 4.74 ± 2.76 ng/ml the next morning (p < 0.05). The mean SAA of a workday showed its lowest value of 59.3 ± 31.5 kU/l during a daytime duty (12:00 – 14:00) and was highest at 112.7 ± 90.1 kU/l at the start of night-time duty (18:00). These salivary biomarkers CORT and SAA on both workdays and holidays showed diurnal variation. In particular, the diurnal variation of CORT followed a well-defined ‘bath-tub’ shaped curve. A negative correlation of –0.36 was observed between the individual’s CORT and SAA in workdays and of –0.62 in holidays, as determined by using the dynamic programming method. It was suggested that the fatigued state of humans is possibly reflected in the diurnal variations of CORT and SAA.