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 th e 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 cyc le is an important consideration. We report a comprehensive study of the di urnal free cortisol cycle designed to analyse its components and to investi gate their reliability and inter-relatedness. We instructed 42 healthy volu nteers to collect saliva samples at home on two consecutive days. On each d ay 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 samples were collected through the day at 3-hour intervals, all synchronized to awakening time. The cortisol r esponse to awakening was calculated in two ways. Overall cortisol productio n 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 re sponse to awakening was calculated as area under the cortisol response curv e (AURC) with reference to the first awakening sample. In addition the unde rlying cortisol secretory activity was assessed based upon the diurnal thre e-hourly samples. All three parameters of adrenocortical activity showed re asonable stability across the two sampling days indicating all were reliabl e indexes of trait characteristic. AUC was representative of underlying diu rnal activity but AURC was not. Measurement at any time point, 3, 6, 9 or 1 2 hours post-awakening was representative of the underlying 12-hour diurnal activity, (C) 2001 Elsevier Science Inc. All rights reserved.