Cortisol secretion is related to electroencephalographic alertness in human subjects during daytime wakefulness

To determine whether human hypothalamo-pituitary-adrenal axis activity is r elated to the alertness level during wakefulness, 10 healthy young men were studied under resting conditions in the daytime (0900-1800 h) after an 8-h nighttime sleep (2300-0700 h). A serial 70-sec gaze fixation task was requ ired every 10 min throughout the daytime experimental session. The correspo nding waking electroencephalographic (EEG) segments were submitted to quant itative spectral analysis, from which EEG beta activity (absolute power den sity in the 13-35 Hz frequency band), an index of central alertness, was co mputed. Blood was collected continuously through an indwelling venous cathe ter and sampled at 10-min intervals. Plasma cortisol concentrations were me asured by RIA, and the corresponding secretory rates were determined by a d econvolution procedure. Analysis of individual profiles demonstrated a declining tendency for EEG b eta activity and cortisol secretory rate, with an overall temporal relation ship indicated by positive and significant cross-correlation coefficients b etween the two variables in all subjects (average r = 0.565, P < 0.001). Ch anges in cortisol secretion lagged behind fluctuations in EEG beta activity , with an average delay of 10 min for all the subjects. On the average, 4.6 +/- 0.4 cortisol secretory pulses and 4.9 +/- 0.5 peaks in EEG beta activi ty were identified by a detection algorithm. A significant, although not sy stematic, association between the episodes in the two variables was found: 44% of the peaks in EEG beta activity (relative amplitude, near 125%; P < 0 .001) occurred during an ascending phase of cortisol secretion, cortisol se cretory rates increasing by 40% (P < 0.01) 10-min after peaks in EEG beta a ctivity. However, no significant change in EEG beta activity was observed d uring the period from 50 min before to 50 min after pulses in cortisol secr etion. In conclusion, the present study describes a temporal coupling between cort isol release and central alertness, as reflected in the waking EEG beta act ivity. These findings suggest the existence of connections between the mech anisms involved in the control of hypothalamo-pituitary-adrenal activity an d the activation processes of the brain, which undergoes varying degrees of alertness throughout daytime wakefulness.