Individual differences in the diurnal cycle of salivary free cortisol: a replication of flattened cycles for some individuals.

Free cortisol measured in saliva has been shown to have the same diurnal rhythm as serum cortisol, one that typically declines rapidly throughout the waking day. A recent study showed that over 15% of a sample of community individuals who were monitored over two days did not show the typical diurnal rhythm. The present study specifically tested the hypothesis that there is significant between-subject variation (individual differences) in diurnal rhythms using multi-level, random regression models. Analyses of participants from four studies were conducted; studies varied in terms of the number of saliva samples taken per day, the number of days studied, and participants' demographic and health status. Significant individual differences of diurnal cycle in each of the four samples were found. In at least 10% of each sample no significant diurnal cycles was detected; however, the overall mean level of cortisol of those with flat cycles differed among the samples. These results suggest that some people do not have the expected diurnal rhythm of cortisol secretion. It is not clear what the determinants of this finding are or if there are any health consequences of having a flat cycle.