Association between time of awakening and diurnal cortisol secretory activity

A 12-hour diurnal profile of salivary free cortisol was measured in healthy adults (n=40) on two consecutive days. Samples were collected at timed int ervals synchronised to awakening. The mean profile is characterised by a ma rked increase in cortisol concentration following awakening, peaking after about 30 min, and a subsequent decline over the remainder of the day. Thus two components of the diurnal cycle were examined: a) the first 45 min post -awakening (the awakening cortisol response) and b) the underlying 12 h pro file from immediately until 12 h post awakening (but without the awakening response). Both of these components were analysed in two ways such as to pr ovide an indication of overall cortisol concentration and the degree of cha nge in cortisol concentration, i.e. the rise for the awakening response and the diurnal decline. Both components of the cortisol diurnal profile were negatively correlated with awakening time. Thus, those subjects who awoke e arliest had higher levels of cortisol over the 45 min following awakening a s well as throughout the rest of the day. They also displayed a more marked diurnal decline to be convergent with late awakeners at the end of diurnal measurement, 12 h following awakening. Hence the diurnal cortisol cycle, w hich is synchronised to awakening, is significantly related to awakening ti me. These findings support the notion of a close association between suprac hiasmatic nucleus (SCN) control of both awakening and cortisol secretory ac tivity. (C) 2001 Elsevier Science Ltd. All rights reserved.