Exploration of the awakening cortisol response in relation to diurnal cortisol secretory activity.

Adrenocortical activity can be assessed by measurement of free cortisol in saliva. Cortisol status has important health implications in both physical and psychological terms. Assessment of cortisol status is complicated by the marked diurnal cortisol cycle. This cycle is characterised by an increase in secretory activity following awakening to achieve the morning acrophase. Thereafter it falls with a declining trend over the remainder of the day. For between subject studies the timing of sampling in relation to this cycle is an important consideration. We report a comprehensive study of the diurnal free cortisol cycle designed to analyse its components and to investigate their reliability and inter-relatedness. We instructed 42 healthy volunteers to collect saliva samples at home on two consecutive days. On each day the first sample was collected immediately upon awakening, followed by a further three samples at 15-minute intervals which collectively comprised the "awakening samples". A further four sample's were collected through the day at 3-hour intervals, all synchronized to awakening time. The cortisol response to awakening was calculated in two ways. Overall cortisol production in the first 45 minutes after awakening was calculated as area under the cortisol curve with reference to zero (AUC). The dynamic of the cortisol response to awakening was calculated as area under the cortisol response curve (AURC) with reference to the first awakening sample. In addition the underlying cortisol secretory activity was assessed based upon the diurnal three-hourly samples. All three parameters of adrenocortical activity showed reasonable stability across the two sampling days indicating all were reliable indexes of trait characteristic. AUC was representative of underlying diurnal activity but AURC was not. Measurement at any time point, 3, 6, 9 or 12 hours post-awakening was representative of the underlying 12-hour diurnal activity.