The purpose of this study was to investigate the relationship between the diurnal pattern of salivary free cortisol to perceived stress and susceptibility to symptoms of upper respiratory illness (URI).
Salivary free cortisol concentration was determined in 34 healthy participants (students) at eight time points, synchronized to awakening, on 2 consecutive days. Participants completed a standard questionnaire to assess perceived stress and subsequently kept a daily record of social proximity and symptoms of upper respiratory illness for 2 weeks.
Participants characterized by consistently larger awakening responses went on to report significantly more URI symptoms. Participants with less pronounced diurnal decline (flatter profiles) reported fewer URI symptoms. The two cortisol components were themselves related and interacted such that participants high on an interactive vector reported approximately three times more URI symptoms than other participants. The URI-associated cortisol components (dynamic changes) were not related to perceived stress, but underlying cortisol secretory activity (overall levels) in the first 45 minutes after awakening was. Dynamic components were, however, related to a social proximity measure, which in turn was related to URI symptoms. Proximity and the interactive cortisol vector together explained a substantial (28%) percentage of the variance in URI symptom reports. The cortisol vector independently and significantly explained 12% of the variation; the proximity measure independently and nonsignificantly contributed 6% of the variation.
URI symptoms were associated with two related dynamic components of the cortisol cycle as determined by synchronization to awakening, whereas stress was related to a measure of overall secretory activity.